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GESTURE NUMBERS

August 8, 2017

Gestures have been used from time immemorial either as a rudimentary number system, or as a supplement to spoken or recorded systems. Claudia Zaslasky (Africa Counts, Lawrence Hill 1999), gives visual examples of a ‘gesture for “six” ’ from Rwanda and a Xhosa woman showing by gesture the number of her children. Zaslasky notes that “in some African societies finger gestures have equal status with the spoken numerals and constitute a proper system of numeration which may or may not agree with the spoken words” (op. cit. p. 37).
I would guess the original ‘symbol’ for zero was something like the double open handed gesture that hunters still use to indicate that they have caught nothing that day. This gesture, common amongst country people in the South of France, does not quite signify “nothing” in the absolute sense, but rather “Nothing where something was to be expected” — which is somewhat different.

“In those systems that build by addition to five, counting usually starts with the little finger of one hand and proceeds by the addition of the appropriate fingers in sequence until five is reached. This number is generally denoted by a closed fist. For six, the little finger of the other hand joins in the counting, and the fingers of the second hand are used in the same sequence as those of the first” (Zaslasky, Africa Counts p. 49).

That gestures directly gave rise to full-scale finger counting seems unlikely : the sophisticated finger counting systems such as the Venerable Bede describes in his 8th century treatise De computo vel loquela digitorum (“On calculating and Speaking with the Fingers”) must surely have developed after an advanced spoken number system. No one in their right senses would use finger counting alone to represent really large quantities : what generally happened is much more likely to have been a combination of various systems, gestures, spoken words, the use of object numbers alongside recorded numerals and so on. Zaslavsky says that the Arusha Masai of Northern Tanzania “rarely give numbers without the accompaniment of finger signs” (op. cit. p. 248). Different ethnic groups had different ‘cut-off points’, most ending with our 50 at most while in the Luo system “there are no gestures for numbers beyond 19” (op. cit. p. 254).
The, at first rather surprising, fact that African languages are predominantly base-five (rather than base-ten) suggests that ‘gestural number systems’ predated written and even spoken ones. Tylor writes: “Word-language not only followed Gesture-language, but actually grew out of it” (Tylor, Primitive Culture)

SH 08/08/17

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Minimal Conceptual Abilities for Development of a Number System

July 29, 2017

 

With one or two possible rare exceptions that will not be dealt with here, animals (including birds, insects &c.) do not use numbers though some primates and birds can be (with difficulty) taught by humans to use them. The reason is quite simple: all species including our own until quite recently (on an evolutionary scale) got along perfectly well without number systems as such. What most, if not all, ‘advanced’ species do have is the ability to make ‘Rough Quantitative Assessments’  (RQA). Rivals for food or females have to decide rapidly whether it’s safer to fight or flee and herbivores of whatever gender have to decide whether one locality has more, or less, nutritious plants. Such assessments usually, implicitly or explicitly, distinguish a threshold: below the threshold it is considered advantageous to fight, above it not. Experience, the great teacher, aids ― or rather obliges ― the species or tribe to hone their rapid assessment abilities since survival and reproductive success in a competitive world may well depend on it. When faced, say, with a threatening group of hungry rivals after you have made a kill (as often occurs), you don’t need to know the exact number of your assailants or their age and state of health ― and trying to work all this out would waste valuable time. You only need to make a snap decision on the information you’ve got ― but this is easier said than done. Subtle though such abilities are, they must be distinguished from numerical procedures.
A very important extension of this ‘RQA’ sense is the ability to notice at once when something important from a set is missing. Primitive peoples so-called regularly astounded explorers or missionaries by their ability to keep check on their packs of dogs, horses or herds of cattle with staggering accuracy (Note 1) even when the culture barely had a number system at all. We still have this ability up to a point but it has been allowed to atrophy because we don’t practise it enough. A schoolteacher generally recognizes at once if a pupil is absent, a collector surveying a roomful of curios at once spots a missing item in a show case and so on. This ability is impressive and doubtless once again of evolutionary importance but it is a ‘pre-numerate’ ability.
To be able to develop a number sense and be capable of manipulating numbers reliably, two ― and as far as I can see only two ― cognitive abilities are required.

The first is the ability to sharply distinguish between ‘one’ and ‘many’, singular and plural, ‘one’ and ‘more-than-one’ (Note 2). But doesn’t everyone have this ability all the time? This is debatable. Some psychologists and philosophers claim that the newborn baby, though perfectly conscious, exists in a completely unified world where no proper distinction is made between itself and its surroundings ― everything is a “buzzing, blooming confusion” (Piaget) but a unified and coherent kind of confusion. According to this view, the great attraction of mysticism is that the practitioner temporarily regains this blissful unitary consciousness ― “Everything is One”. Certainly, it would seem that there can be no awareness of the ‘I’ without awareness of the ‘non-I’. Interestingly, in at least one ancient language, the word for ‘one’ or ‘single’ is the same as the word for ‘alone’ (Note 3).

The second absolutely essential ability for number development is the ability to ‘pair off’  two collections of objects. This ability does not come naturally and primary schoolteachers often have great difficulty in getting tiny children to develop it ― apart from anything else, it seems a rather pointless thing to do. So what, if we can line up two groups of apples (or boys and girls) so that each apple, or boy or girl, from one group is paired off exactly with an apple, or boy or girl, from the other group? Modern mathematicians call this carrying out a ‘One-One Correspondence’ and it is only since the latter 19th century that mathematicians have realized it is the key to understanding numbering.
It is important to note that the child may perfectly well be able to form correctly two sets of paired similar objects, for example pens taken from a pool of pens, but be unable, or refuse, to pair off apples and pens.
Not only children but whole cultures strongly resisted the idea that any set of ‘ones’ ― i.e. discrete objects that do not fuse when brought close together ― can be ‘paired off’ with any other set of ‘ones’ (provided, of course, that there are enough objects in the second set). Several societies, when they did eventually develop spoken and written numerals, had more than one set depending on what sort of ‘things’ were being compared: the Nootka of British Columbia, for example, went so far as to use different number words for rounded objects and long, thin objects(Note 4). Other cultures, understandably, considered it blasphemous to use the same number words or number signs for humans as for gods, which is one reason why the Mayans had three different sets of numerals. Even in our own books, until recently, the date of publication was always given in Roman numerals ― as if ‘years’ somehow required a different set of numerals to everything else. More generally, even today, there is the persistent feeling that there is something degrading and dehumanizing about humans being numbered in the same way as cattle or pieces of wood. Accepting that two sets of objects, no matter what the objects are, provided  they can be exactly paired off, item for item, are ‘numerically equivalent’, ‘represent the same number’, involves making a giant conceptual leap that we still baulk at.

Note 1 : “It was related by a missionary to the Abipones, a tribe of South American Indians compelled by a shortage of food to migrate (in the 18th century): “The long train of mounted women was surrounded in front, in the rear, and on both sides by countless numbers of dogs. From their saddles the Indians would look around and inspect them. If so much as a single dog was missing from the huge pack, they would keep calling until all were collected together again.” (…) Yet they [the Abipones] had only three number words and showed the strongest resistance to learning the number sequence from white men.
(…) We can understand such phenomena if we remember the far closer relationship of these people with the world around them: the keen observation that unhesitatingly notes the absence of a single animal and can say which one is missing, and the translation of a number that cannot be visualized into a clearly perceived spatial form.”        Menninger, Number Words and Number Symbols p. 11

Note 2 : “A few other South American languages are almost equally destitute of pure numerical words. But even here, rudimentary as the number sense undoubtedly is, it is not wholly lacking; and some indirect expression, or some form of circumlocution, shows a conception of the difference between one and two, or at least, between one and many” (Courant, The Number Concept Its Origin and Development p. 5)

Note 3 : “The Tacanas of the same country [Bolivia] have no real numerals whatever, but expressed their idea for “one” by the word etama, meaning alone” (Courant, The Number Concept Its Origin and Development p.5)

Note 4 : See The Calendrical and Numerical Systems of the Nootka by William Folan in Native American Mathematics edited by Michael Closs.

 

 

 

EUCLID AND UNIQUE PRIME FACTORISATION

May 30, 2017

It is often said that Euclid (who devoted Books VII – XI of his Elements to Number Theory) recognized the importance of Unique Factorization into Primes and established it by a theorem (Proposition 14 of Book IX). This is not quite correct. Modern authors usually present UPF in the following way

THEOREM Any positive integer N can be written as a product of primes in one and only one way barring changes in order. i.e.  N = pa qb rc…..

        But what Euclid establishes by proving Book IX Proposition 14 — Heath, whose translation I use throughout, calls ‘theorems’ ‘propositions’ — is rather less than this, viz.

“If a number be the least that is measured by prime numbers, it will not be measured by any other prime number except those originally measuring it.”

Now, from this one can, with the help of one or two other theorems,   deduce Unique Prime Factorization (UPF), but Euclid does not actually do this. For one thing, Euclid would need to show that every (natural) number can be presented as a product of primes if Proposition 14 is to have a universal application. He goes some way to doing this in Propositions 31 and 32 of Book VII : Any composite number is measured by some prime number” and “Any number either is prime or is measured by some prime number”. But, for some reason, we lack the clinching Proposition, that all numbers can be written as a product of primes and that there is only one way of doing this barring changes in order.

Euclid’s presentation of Number Theory is so idiosyncratic, not to say perverse, that many readers, flipping through the Elements,  do not even realize that he ever dealt with numbers at all. This is because Euclid insists on presenting (whole) numbers as line segments A ______________       B __________  and not, as one would expect, as collections of discrete elements, e.g. by such sequences as ● ● ● ● ● ● ● ●  or □ □ □ □  It is true that, by presenting numbers as lines Euclid gains generality : we can see in the above that A > B but we are not limited to specific magnitudes. Also, unlike us, Euclid did not have the mathematical etcetera symbol, ….

However, I doubt if this was the real reason. By Euclid’s time geometry had almost entirely ousted arithmetic as the dominant branch of mathematics much in the way that algebra subsequently ousted geometry. Pride of place in the Elements is given to the theory of proportion developed by Eudoxus. In the books devoted to Number Theory Euclid only deals with whole numbers (always imaged by line segments) and ratios between whole numbers which imitate  ratios between sides of triangles and other figures. He does not mention ‘fractions’ as such though Greek housewives and practical people must have been well acquainted with them. Why this emphasis on geometry even when it is inappropriate?  Part of the blame, if blame it is, must be assigned  to Plato who, though not himself a mathematician, was well versed in the higher mathematics of his time and remains one of the most important theorists in the whole history of mathematics. Plato’s view that the ‘truths of mathematics’ are in some sense independent of human experience, while nonetheless underlying it, is the view still held by  most pure mathematicians today. Plato considered mere calculation with numbers to be a lowly activity, the ‘affair of craftsmen and tradesmen’, while geometry was a discipline that ennobled the practitioner by fixing his eye on the eternal. Hence the  radical ‘geometrization’ of number that we find in Euclid.

In his Books on Number Theory, one suspects that Euclid was building on a much older arithmetic tradition which not only presented numbers as discrete entities but actually used objects such as pebbles or shells in calculations and formed them into shapes — which is why we still speak of ‘triangular numbers’, ‘square numbers’ and so forth (Note 1). The material of Book VII, the basic Book dealing with Number Theory, looks as if it goes back a very long way indeed and this  is at once an advantage and a drawback.

It is an advantage because Euclid kicks off with an eminently practical procedure (rather than an abstract theorem), the so-called Euclidian Algorithm, and makes it the foundation of the entire edifice. Most of Euclid’s proofs are by contradiction and thus ‘non-constructive’  but the Euclidian Algorithm not only demonstrates that a ‘least common measure’ of two or more numbers always exists, but actually shows you how to obtain it. Remarkably, the Euclidian Algorithm works perfectly well in any base, or indeed without any base at all — and this alone suggests that it is a very ancient procedure. It was quite possibly  discovered before written numbers even existed : in effect, it shows you how to group or bag up two different collections of similarly sized  objects (such as beads or shells) without anything being left over while  using the largest possible bag size. Proposition 1 is a special case of this : when the largest bag size possible turns out to be the unit. Such an outcome must have seemed extraordinary to the people who first discovered it, and indeed mankind has ever since been fascinated by ‘prime numbers’ — they were originally called ‘line numbers’ because they could only be laid out in a line or column, never as a rectangle.

However, probably because they are based on an ancient source, Euclid’s presentation in the Books devoted to Number Theory is not  so impeccably logical as in the other Books. Euclid does not introduce any new Axioms in Book VII, the first of the four books dealing with Number Theory, though he does give twenty-two Definitions. He presumably  assumed that the general Axioms, given in Book I, suffice. In fact, they do not. Operations with or on numbers differ from operations on geometric figures since plane figures and solids do not have ‘factors’ in the way that numbers do. As Heath notes, Euclid does not state as an Axiom that factorisation is transitive (as we would put it), i.e. “If a / B & B/ C, then a/C”, nor does he prove it as a theorem though he assumes it throughout. The Euclidian Algorithm would not work without this feature and a large number of other Propositions would be defective. Indeed, as Heath specifies, we not only need the above but the Sum and Difference Factorisation Theorems which, in Euclid’s parlance, would be

If A measures B, and also measures C, then A measures the sum of B and C, also the difference of B and C when they are unequal and B is greater than C.

        An even more serious admission, from our point of view, is that Euclid does not explicitly state the Well-Ordering Principle, namely that Every non-increasing sequence of natural numbers has a least member though he assumes it in various propositions. Given the strong anti-infinity bias of Greek thought, Euclid would doubtless have thought it unnecessary.

Euclid proves Proposition 14 (If a number be the least that is measured by prime numbers, it will not be measured by any other prime number except those originally measuring it) in the following way :

“Let N = pqrs… where  p, q, r…. are primes. Suppose a prime u different from primes p, q, r… and which divides N. Then N = u × b.
But if any prime number divides (m × n) and does not divide m, it must divide n [VII. 30].
Now, p divides N and p does not divide u since u, p are primes and u ≠p Therefore, p divides b. And the same applies to q, r….
Therefore, pqr…  divides b
        But this is contrary to the hypothesis, since b < N and N is the smallest number that can be divided by pqr….
        Therefore, N has no prime factors apart from p, q, r…

It should be noted that this is a Proof by Contradiction and that it applies only to the case where p, q, r… are each of them distinct primes.

What Propositions does this proof rely on?
Firstly, on VII. Proposition 30 “If two numbers by multiplying one another make some number, and any prime number measure the product, it will also measure one of the original numbers.”

This is one of the most important theorems in the whole of Number Theory and I call it the Prime Factor Theorem. What applies here is the special case when one at least of the two original numbers is prime — and a different prime from the ‘dividing number’.
But Euclid also needs to prove, or to have proved, that N really is, in our terms, the Least Common Multiple of p, q , r…. This he does in Book VII. Propositions 34 and 35 which detail the procedure for finding the Least Common Multiple, first of two numbers (Prop. 35), and secondly of three or more numbers (Prop. 36). As a special case, Euclid shows that the LCM of two numbers a, b that are prime to each other is ab and that the procedure can be applied as many times as we wish so that the LCM of a,b,c…. where a, b, c are all primes is abc… He is also scrupulous enough to show (Proposition 29) that a prime and any other ‘number it does not measure’ are prime to each other, which makes any two primes ‘prime to each other’.
Euclid does not generalize Proposition 14 to powers of these primes, i.e. to our pa qb rc…  though this extension is in effect covered by the propositions about Least Common Multiples VII. 34, 35 and 36 taken together with VII. 31 and 32.

The propositions concerning LCMs are very much what one would expect and are easily assented to. The same does not apply to the Prime Factor Theorem which is by no means ‘intuitively obvious’ nor especially easy to establish.
In modern terms Euclid’s proof of the Prime Side Theorem is as follows:
“Suppose p divides N (= ab) where p is prime, and p does not divide a.
Then (p, a) = 1 [VII. 29]
Let ab = pm = N where m is some number.
Then p/a = b/m  [VII. 19]
But since (p, a) = 1, p/a is in its lowest terms. Therefore m must be a multiple of a and b a multiple of p [VII. 20, 21].

So, if p divides ab where p is prime, then either p divides a or p divides b (or both).”

         The key proposition here is VII. 19, the Cross Ratio Theorem: “If four numbers be proportional, the number produced from the first and fourth will be equal to the number produced from the second and third; and, if the number produced from the first and fourth be equal to that produced from the second and third, the four numbers will be proportional.”  

This cumbersome statement shows the importance of algebraic notation which the Greeks did not have. Remember that Euclid is speaking only of ratios between hypothetical line segments, not of ‘rational numbers’ as modern mathematicians understand them. However, bearing this in mind, Euclid’s proof may be presented thus :
“Let ac/ad =  c/d = a/b
       But a/b = ac/bc
So ac/ad =  ac/bc
But this can only be true if ad = bc

Conversely, let ad = bc
     Then ac/ad = ac/bc
However, ac/ad = c/d
     Also ac/bc = a/b
Therefore a/b = c/d

The above itself depends on the legitimacy of ‘cancelling out’, likewise the legitimacy of multiplying and dividing numerator and denominator by the same factor. Euclid has already dealt with such issues and I will not trace the derivation any further back. He has, I think, made a proposition by no means obvious — the ‘Prime Factor Theorem’ — entirely acceptable and, if we accept the latter, then seemingly we must accept Book IX Proposition 14. Apart from some tidying up and expansion, Unique Prime Factorization in the Natural Numbers has been established.

                                                                                                                                                                                                                         SH       30/05/17

 Note 1 : “It seems clear that the oldest Pythagoreans were acquainted with the formation of triangular and square numbers by means of pebbles or dots; and we judge from the account in Speusippus’s book On the Pythagorean Numbers, which was based on the works of Philolaus, that the latter dealt with linear numbers, polygonal numbers, and plane, and solid numbers of all sorts….”   (Heath, History of Greek Mathematics p. 76)

 

MULTIPLICATION

March 8, 2015

Multiplication is a nonsense interpreted literally. ‘Increase and multiply’ ― but if you multiply something once it stays the same! And one could very reasonably conclude that if you get no change when you carry out an operation once, you will get no change when you repeat it: so no matter how many times you ‘multiply’ something it does not get any bigger. Abraham would never have had as many descendants as there are stars in the sky going about things this way.

Why is it, then, that we have this strange rule that N × 1 = N? The number system we work with today is formalised in terms of the Axioms of Fields: we have two basic operations ‘+’ and ‘×’ and both have a so-called ‘identity element’ which keeps things as they are, 0 for addition and 1 for multiplication. Why not have zero as the null operation for multiplication as well? On the face of it this makes a lot more sense since abstaining from increasing (or decreasing) something means it stays the same. In such a system

1 × 0 = 1 and more generally

N × 0 = N where N is a positive integer.

The most natural sense of ‘multiplication’ is doubling what you already have, so

1 × 1 = 2  and more generally

N × 1 = 2N  

It is not clear how we should proceed from now on. What happens when you have N × 2 ?  We could interpret this as an instruction to double again in which case multiplication becomes the initial quantity ‘times’ the appropriate power of 2. But this lacks generality. Better to fall back on defining multiplication as repeated addition and interpret the ‘×’ as an instruction to “add on another N” or

N × 2 = 3N and more generally N × m = (m + 1)N 

        This sort of multiplication does not, as far as I can see, lead to contradiction and I have even attempted to use it. But it is extremely inconvenient because we lose the so-called commutativity of ‘×’ as an operation: the result would not generally be the same if we invert the two numbers involved. 3 × 1 in this maverick system gives 6 but 1 × 3 gives 4. What we do get in lieu is the peculiar

N × m = m × N if and only if N = (m ─ 1)  so that
(4 × 5) = (5 × 4) = 20

This type of multiplication would also cause problems when related to ‘normal’ division since
(N × m) /m  ≠ N  
in general. It thus requires a re-definition of division. And so on.

It would seem unlikely that these formal issues were the original reason for the rule that a single multiplication leaves the quantity unchanged. Arithmetic has only been formalised during the last 150 years or so while people have been handling numbers for thousands of years. There are two plausible explanations for the ‘null multiplication rule’.

It is generally accepted today that the earliest type of arithmetic was done using the fingers, sometimes the toes as well. This is shown by the abundance of number words that are related to the fingers: ‘digit, for example, comes from the Latin digitus meaning finger. And the widespread use of base 10 throughout the world rather than the much more convenient 12 is doubtless due to the anatomical accident whereby mammals have ten fingers and thumbs rather than twelve. Finger counting and, more generally, ‘finger arithmetic’ was once widespread since most of the world was illiterate and was allegedly still used within living memory by pearl traders in the Middle East. The Venerable Bede wrote a treatise on finger counting and “the reader will be surprised  to find that underlying these finger  gestures is a positional or place-value system” (Menninger. Number Words).

Now I have actually experimented with a simple finger arithmetical system. Numbers, abstract, gestural or concrete, were not originally invented for ‘doing equations’ but in order to assess, and perhaps subsequently record, quantities of objects by representing them in a standard symbolic form ― even today numbers are used primarily for the recording of data. If you are walking along and want to assess how many trees there are in a clump you cannot operate with the trees since they are fixed. What you can do is to match each sub-clump of trees with the fingers of one hand and then use the other hand to record the number of handfuls if there aren’t too many. The eventual quantity can then be memorised and, if required, be subsequently recorded in a more permanent manner by way of charcoal marks on a wall, scratches on a bone, knots in a rope and so on — a Roman would have had a household slave with him holding a portable marble abacus.

Now such a procedure involves both division and multiplication. The collection of real objects is first of all ‘divided up’, at least in imagination,  into so many fives or tens and each batch is ‘multiplied’ by repeatedly showing two open hands and the remainder neglected or shown with a single  hand. There is, however, a significant difference between the two operations: it is the collection itself (the trees or beans or warriors) that is first ‘divided up’ into so many tens, but it is the copy, the handful, that is ‘multiplied’. This seems to be the true meaning of ‘multiplication’, namely ‘replication’ or ‘identical copying’ (cloning) and this, of course, is how mRNA goes about its business when it copies part of a strand of DNA in the nucleus while the actual assembly of amino-acids to form proteins takes place later outside the nucleus.

In the context of finger assessment or DNA replication, the rules for multiplication and division make perfect sense. The first copying is, as it were, an inert operation while ‘copying the copy’ by repetition is creative. The basic point is that the original collection being represented, the clump of trees or the group of men or the bases making up a gene, is not part of the arithmetic operation proper: it  functions as a sort of template. And when we pass on to abstract operations where there is not necessarily a real object or collection in view, we retain the same mental picture to guide our operations. There is an original numerical quantity which is ‘out there’: we represent it by some written or verbal symbol and from then on operate on that.

Primitive commercial practice would have reinforced this schema. Arithmetic only got going with the rise of the large Middle Eastern empires, Assyria, Babylon and the like, when trade was extensive and an extensive bureaucracy was in place. It has been suggested that the development of writing, in the form originally of some sort of ideogram or recognizable picture, came about because of trade. Merchandise, say combs or olives or pins, were apparently often transported in sealed containers which could be checked on arrival to see if they had been tampered with. But how to know what was inside without breaking the seal? A simple stratagem would be to have a clay model of the object attached to the outside of the container indicating the contents. Later, a picture of the object replaced the clay model and later still a stylised representation and eventually a ‘word’. This symbol is then ‘multiplied’ so many times to indicate the sum total of the contents. Again, we have the strict separation between the representation and the actual object or objects without which the system would not work.

SH   8/04/15

 

Even and Odd

March 5, 2015

Ο                                   ΟΟ
ΟΟΟ                            ΟΟΟΟ
ΟΟΟΟΟ                      ΟΟΟΟΟΟ
ΟΟΟΟΟΟΟ                ΟΟΟΟΟΟΟΟ

Animals and so-called primitive peoples do not bother to make nice distinctions between entities on the basis of number and even today, when  deprived of technological aids, we are not at all good at it (Note 1).  What people do ‘naturally’ is to make distinctions of type not number and the favourite principle of division by type is the two-valued either/or principle.  Plato thought that this principle, dichotomy, was so fundamental that all knowledge was based on it — the reason for this being because the brain works in this way, the nerve synapsis is either ‘on’ or ‘off’. Psychologically human beings have a very strong inclination to proceed by straight two-valued distinctions, light/dark, this/that, on/off, sacred/profane, Greek/Barbarian, Jew/Gentile, good/evil and so on — more complex gradations are only introduced later and usually with great reluctance Science has eventually recognized the complexity of nature and apart from gender there are not many true scientific dichotomies left though we still have the classification of animals into  vertebrates and invertebrates.

Numbers themselves very early on got classified into even and odd , the most fundamental numerical distinction after the classification one and many which is even more basic.

The classification even/odd is radical: it provides what modern mathematicians call a partition of the whole set. That is, the classification principle is exhaustive : with the possible exception of the unit, all  numbers fall into one or other of the two categories. Moreover, the two classes are mutually exclusive: no number appearing in the list of evens will appear in thelist of odds. This is by no means true of all classification principles for numbers as one might perhaps at first assume. Numbers can be classified, for example, as triangular and as rectangular according to whether they can be (literally) made into rectangles or equilateral triangles. But ΟΟΟΟΟΟ turns out to be both since it can be formed either into a triangle or a rectangle:

ΟΟΟ                                         ΟΟΟ
ΟΟΟ                                         ΟΟ
Ο
The Greeks, like practically all cultures in the ancient world, viewed the odd and even numbers as male and female respectively — presumably because a woman has ‘two’ breasts and a male only one penis. And, since oddness, though in Greek the term did not have the same associations as in English, was nonetheless defined with respect to evenness and not the reverse, this made an odd number a sort of female manqué. This must have posed a problem for their strongly patriarchal society but the Greek philosophers and mathematicians got round this by arguing that ‘one’  (and not ‘two’) was the basis of the number system while ‘one’ was the ‘father of all numbers’.

On the other hand a matriarchal society or a species where females were dominant would almost certainly, and with better reasoning, have made ‘one’ a female number, the primeval egg from which the whole numerical progeny emerged. Those who consider that mathematics is in some sense ‘eternally true’ should reflect on the question of how mathematics would  have developed within a hermaphroditic species, or in a world where there were three and not two humanoid genders as in Ian Banks’s science-fiction novel  The Player of Games.

Evenness is not easy to define — nor for that matter to recognize as I have just realized since, coming across an earlier version of this section, I found I was momentarily incapable of deciding which of the rows of balls pictured at the head of this chapter represented odd or even numbers. We have to appeal to some very basic feeling for ‘symmetry’ — what is on one side of a dividing line is exactly matched by what is on the other side of it. A definition could thus be

If you can pair off a collection within itself and nothing remains over, then the collection is called even, if you cannot do this the collection is termed odd.

This makes oddness anomalous and less basic than evenness which intuitively one feels to be right —  we would not, I think, ever dream of defining oddness and then say “If a collection is not odd, it is even”. And although it is only in English and a few other languages that ‘odd’ also means ‘strange’, the pejorative sense that the word odd has picked up suggests that we expect and desire things to match up, i.e. we expect, or at least desire, them  to be ‘even’ —  the figure of Justice holds a pair of evenly balanced scales.

The sense of even as ‘level’ may well be the original one. If we have two collections of objects which, individually,  are more or less identical, then a pair of scales remains level if the collections are placed on each arm of the lever (at the same distance).  One could define even and odd thus pragmatically:

“If a collection of identical standard objects can be divided up in a way which keeps the arms of a balance level, then the collection is termed even. If this is not possible it is termed odd.”

This definition avoids using the word two which is preferable since the sense of things being ‘even’ is much more fundamental than a feeling for ‘twoness’  — for this reason the distinction even/odd, like the even more fundamental ‘one/many’ , belongs to the stage of pre-numbering rather than that of numbering.

Early man would not have had a pair of scales, of course, but he would have been familiar with the procedure of ‘equal division’, and the simplest way of dividing up a collection of objects is to separate it into two equal parts. If there was an item left over it could simply be thrown away. Evenness is thus not only the simplest way of dividing up a set of objects but the principle of division which makes the remainder a minimum: any other method of division  runs the risk of having more objects left over.

Euclid’s definition is that of equal division. He says “An even number is that which is divisible into two equal parts” (Elements Definition 6. Book VII)  and “An odd number is that which is not divisible into two equal parts, or that which differs by  a unit from an even number”  (Elements  Definition 7. Book VII). Incidentally, in Euclid ‘number’ not only always has the sense ‘positive integer but has a concrete sense — he defines ‘number‘ as a “multitude composed of units”.

Note that Euclid defines odd first privatively (by what it is not) and then as something deficient with reference to an even number. The second definition is still with us today: algebraically the formula for the odd numbers is (2n-1) where n is given the successive values 1, 2, 3…. or sometimes (in order to leave 1 out of it) by giving n the successive values 2, 3, 4….  In concrete terms,  we have the sequence

Ο     ΟΟ     ΟΟΟ  ……..                 …..

Duplicating them gives us the ‘doubles’ or even numbers

Ο     ΟΟ     ΟΟΟ  ..….
Ο     ΟΟ     ΟΟΟ  ……

and  removing a unit each time gives us the ‘deficient’ odd numbers.

The unit itself is something out on its own and was traditionally regarded as  neither even nor odd. It is certainly not even according to the ‘equal division’ definition since it cannot be divided at all (within the context of whole number theory) and it cannot be put on the scales without disturbing equilibrium. In practice it is often convenient to treat the unit as if it were odd, just as it is to consider it a square number, cube number and so forth, otherwise many theorems would have to be stated twice over. Context usually makes it clear whether the term ‘number’ includes the unit or not.

Note that distinguishing between even and odd has nothing to do with counting or even with distinguishing between greater or less – knowing that a number is even tells you nothing about its size. And vice-versa, associating a number word or symbol with a collection of objects will not inform you as to  whether the quantity is even or odd — there are no ‘even’ or ‘odd’ endings to the spoken word like those showing whether something is singular or plural,  masculine or feminine.

It is significant that we do not have words for numbers which, for example, are multiples of four or which leave a remainder of one unit when divided into three. (The Greek mathematicians did, however, speak of ‘even-even’ numbers.) If our species had three genders instead of two, as in the world described in The Player of Games, we would maybe tend to divide things into threes and classify all numbers according to whether they could be divided into three parts exactly, were a counter short or a counter over. This, however, would have made things so much more complicated that such a species would most likely have taken even longer to develop numbering and arithmetic than in our own case.

The distinction even/odd is the first and simplest case of what is today called a congruence. The integers can be separated out into so-called equivalence classes according to the remainder left when they are divided by a given number termed the modulus. All numbers are in the same class (modulus 1) since when they are separated out into ones there is only one possible remainder : nothing at all. In Gauss’s notation the even numbers are the numbers which leave a remainder of zero when divided by 2, or are ‘0 (mod 2)’ where mod is short for modulus. And the odd numbers are all 1 (mod 2) i.e. leave a unit when separated into twos. What is striking is that although the distinction between even and odd, i.e. distinction between numbers that are 0 or 1 (mod 2) is prehistoric, congruence arithmetic as such was invented by Gauss a mere couple of centuries ago.

In concrete terms we can set up equivalence classes relative to a given modulus by arranging collections of counters (in fact or in imagination) between parallel lines of set width starting with unit width, then a width which allows two counters only, then three and so on. This image enables us to see at once that the sum of any two or more even numbers is always even.

And since an odd number has an extra  Ο  this means a pair of odd numbers have each an extra unit and so, if we fit them together to make the units face each other we have an even result. Thus    Even plus even equals even” and “Odd plus odd equals even” are not just jingles we have to learn at school but correspond to what actually happens if we try to arrange actual counters or squares so that they match up.

We end up with the following two tables which may well have been the earliest ones ever to have been drawn up by mathematicians.

­­­­­­­­­­­­­­­­­­­­­­­          +       odd      even                        ×     odd    even  

       odd      even    odd                     odd    odd    even

       even    odd    even                     even  even  even

 

All this may seem so obvious that it is hardly worth stating but simply by appealing to these tables many results can be deduced that are far from being self-evident. For example, we find by experience that certain concrete  numbers can be arranged as rectangles and that, amongst these rectangular numbers, there are ones that can be separated into two smaller rectangles and those that cannot be. However if I am told that a certain collection can be arranged as a rectangle with one side just a unit greater than the other, then I can immediately deduce that it can be separated into two smaller rectangles. Why am I so sure of this? Because, referring to the tables above,

1.) the ‘product’ of an even and an odd number is even;
2.) an even number can by definition always be separated into two equal parts.

           I could deduce this even if I was a member of a society which had no written number system and no more than a handful of number words.

This is only the beginning: the banal distinction between even and odd and reference to the entries in the tables above crops up in a surprising amount of proofs in number theory. The famous proof that the square root of 2 is not a rational number — as we would put it — is based on the fact that no quantity made up of so many equal bits can be at once even and odd.                                                                       SH 5/03/15

 

Note 1  This fact (that human beings are not naturally very good at assessing numerical quantity) is paradoxical since mankind is the numerical animal par excellence. Mathematics is the classic case of the weakling who makes himself into Arnold Schwarzenegger. It is because we are so bad at quantitative assessment that playing cards are obliged to show the number words in the corner of the card and why the dots on a dice are arranged in set patterns to avoid confusion.

 

 

Number Conservation Principle

March 3, 2015

There can be no doubt that mathematics was developed historically for very pragmatic and unromantic reasons. It was the Middle Eastern hierarchic societies, especially Babylon and Egypt, who not only started arithmetic as we understand it but took it to a surprisingly high level of development : the former, for example, gave a good estimate for the square root of 2 while the Egyptian scribes were adept at handling fractions. Such societies could not exist without efficient centralized planning, standardized weights and measures, effective methods of taxation and ‘fair’ remuneration of officials : indeed, they are not so very different from the EEC today !  This is not to say that the scribes and State officials did not have a ‘pure mathematical’ interest as well : like Civil Servants doing Sudoku in the morning break, their distant predecessors seemed to have enjoyed mathematical puzzles, as the Rhine papyrus shows.

The Greeks cast this mass of sporadic data, methods and formulae into a rigorous axiomatic mould with the results we know. However, in his Elements of Geometry, Euclid still has his eye on figures that can actually be drawn and many of the so-called ‘theorems’ (which Heath, Euclid’s best English translator, calls ‘propositions’) are better described as ‘Procedures’. For example, the very first ‘Proposition’ of Book I is “[how ] On a given straight line to construct an equilateral triangle”.

The success of Euclid’s Elements and the axiomatic method in general meant that many post-Renaissance early scientists attempted to cast their subject in the same mould and contemporary works on (classical) Mechanics and Thermo-dynamics are still written somewhat in this style. Newton in particular loosely imitated Euclid in his Principia but it is important not to see this as a retreat into some transcendental Platonic realm of pure mathematics. Notwithstanding the complicated mathematical formalism, Newton’s system of Mechanics was rooted in human sense experience, universal human experience, although it systematised, extended and idealised this experience in various ways. His audience, even those who were illiterate, could be expected to know what a ‘solid body’ was, what a ‘force’ was and even the somewhat metaphysical notion of ‘mass’ was not so very far fetched when defined in Newtonian terms as “the quantity of matter within a body”. Children know that if you push an object it usually moves in the direction of the push (‘applied force’), though they do not perhaps “know that they know this”. And although bowls on a bowling green and bar billiards are modern games, people have been playing around with balls from time immemorial and know from experience that if you strike something from the side rather than bang on from the back, it moves off at a slant. As far as we know, it was Leonardo da Vinci who first gave us the well-known diagram of the parallelogram of forces (in his Notebooks), and Newton who put the problem on a truly scientific footing by the notion of ‘resultant’ force — but this abstract treatment was entirely intelligible and by and large convincing to anyone who had mucked around with solid bodies.

The accepted procedure in such subjects, following Euclid, is to start off with certain ‘Axioms’, ‘Postulates’ and ‘Definitions’ and then proceed to derive conclusions, the ‘theorems’. For Euclid there was a difference between an ‘Axiom’ and a ‘Postulate’ : the former was an entirely general principle (Heath translates the Greek as ‘Common Notion’), while a ‘postulate’ had a more technical and constructive character. Thus, Euclid takes as one of his Axioms, Things which are equal to the same thing are equal to each other (Heath’s translation). In modern terms, Euclid is asserting the ‘transitivity’ of the ‘equality relation’ : something that is in a sense ‘obvious’ once it has been stated, but is well worth stating nonetheless. But the Postulates are introduced by “Let the following be postulated” and the first one is : 1. To draw a straight line from any point to any point. This doesn’t sound very good English but it is, I think, evident what Euclid has in mind. Practically, it may well be that I cannot ‘draw a straight line from point A to point B’ because the ‘points’ are too far apart, or one or both are inaccessible. But from a mathematical point of view, we need to assume that we can do this, and this assumption needs to be stated. In other words, any geometric conclusions we draw remain valid because the only reasons stopping us actually testing a particular claim are purely technical. This is all very sensible and, from a mathematical point of view, necessary. It does not mean that we can, or believe that anyone ever could, ‘draw a line from here to the Sun’, but that is not sufficient reason to stop us drawing certain conclusions which, hopefully, we can test in more mundane cases : it does not necessarily involve us in any supposed Platonic belief in a timeless world of Forms. Nonetheless, it was this gap between the observed and the imagined that, when it widened still further, started dissociating mathematics from the physical world, a process that has now gone the whole way in a manner that even Plato might not have approved.

Though this aspect is not so evident in Euclid, as the natural sciences developed in the West, it became necessary to make it very clear to what sort of entities the ‘principles’ and deductions therefrom applied. For example, Newtonian Mechanics only applied to ‘objects’, not (necessarily) to human beings in their entirety, and in modern times it became necessary to go even further and make it clear that Newtonian Mechanics only applied to relatively large massive bodies moving at modest speeds relative to each other (modest compared to the speed of light).

Most people would be surprised to learn that Euclid devoted four books of his Elements to Number Theory (Books VII – X). Though containing many important theorems, these Books are not quite so rigorous as the strictly geometric ones and they strike the modern reader as being quite perverse in their presentationof numbers as line segments instead of collections of discrete objects, blobs or squares say. Euclid was building on the earlier work of more ‘primitive’ Number theorists who actually worked with stones and pebbles, hence the interest in the visual appearance of numbers, in ‘square numbers’, i.e. collections of objects that can be made into a square, ‘triangular numbers’ and so forth. Also, there are, as Heath remarks, certain important ‘Common Notions’ (Axioms) that are not expressed such as the ‘transitivity’ of divisibility, as modern mathematicians would put it, i.e. if a number ‘goes into’ another exactly (‘measures it’), then it also ‘goes into’ any multiple of that number. Again, this is something we take for granted but which, for all that, is worth mentioning.

Euclid’s treatment of Numbers is, thus, already abstract and geometrical compared to what we surmise was the earlier approach. Today, ‘numbers’ are defined in a completely abstract way, so abstract that they are unrecognizable as such to the ordinary person. The only person I know of who in fairly recent times dared to treat mathematics, or at least arithmetic, in an empirical manner was John Stuart Mill, with the result that he has been pilloried ever since by Frege,  Russell and more or less everyone else who has written on the foundations of mathematics. The philosopher Mackie once asked disingenuously, “Why cannot we have an empirical mathematics?” but, as far as I know, made no attempts to create one.

I believe that arithmetic and the theory of numbers can, and should, be presented as a science. So, what does this science depend on? Today, a large amount of classical physics is made to depend on conservation laws, themselves extensions of Newton’s Laws, thus we have the Conservation of Momentum, the Conservation of Angular Momentum, and even in an era where very little can ne taken for granted in physics, the Principle of the Conservation of Mass/Energy is still just about standing up. Now any completely generalised ‘principle’, though it can be shown to be ‘wrong’ or at least inappropriate in certain circumstances, can obviously never be justified completely : the validity of such principles is, firstly that they fit a good deal of the data we already have, simplify and make more intelligible the world around us and permit prediction which can in special cases be tested. Sometimes, the ‘principles’ are simply necessities, sine qua nons,m without which we just could not get started at all. For example, in physics, we usually have to assume that, given equivalent conditions, we will get equivalent results in a particular experiment, even though this is by no means self-evident and, if Quantum Mechanics is believed, is not strictly true!

Is there a key principle on which everything about numbers relies?  Yes, I believe there is. It is what I call the Number Conservation Principle and it is made up of two sub-Principles, the Principle of Replacement and the Disordering Principle :

Principle of Replacement

The numerical status of a collection of objects is not changed if each individual object is replaced by a different individual object.

Disordering Principle

The numerical status of a collection of objects is not changed by rearrangement so long as no object is created or destroyed.

I think most people would agree, if they can accept the somewhat portentous language, that this is how things are, that the Principles are true. You think there are ‘seven’ objects on the table. I tell you to close your eyes and if, when you open them again, every previous object has been replaced by a different one, you will nonetheless (I hope) still say there are ‘seven’ objects on the table. Similarly, if a completely change the arrangement, scattering the objects around (while taking care that none falls off the table), there will still be the same ‘amount’ of objects. Also, I can do these two operations in an y order and as many times as I like, and still ‘something’, what I call the ‘numerical status’ of the collection has not changed.    

The formulation is open to the objection that both sub-Principles are stated in the negative: “is not changed”. However, it is perhaps impossible to avoid this since ‘number’, whatever it is, results from an “ignoring of differential qualities” as Piaget and Imfeld put it so well. ‘Number’ is what ‘is left’ when you have thrown away all distinctions of size, colour, race, weight, attractiveness, gender and so forth, and still have something left that is worth having or stating.

It should be emphasized that it is only when children in the Primary School have ‘understood’ the Number Conservation Principle that they are considered to have begun to be numerate : if they do not accept it, they will be classed as children with special needs. Neither the children, nor most likely the Primary Schoolteacher herself, have heard of the Peano Axioms, or the Axioms of Zermelo-Fraenkel Set Theory, but that does not stop them having made a beginning in ‘understanding number’. Zermelo and Fraenkel themselves had to go through this particular mill.
        In other cultures different bases were used depending on the different objects being counted. Flat objects like cloths were counted by the Aztecs in twenties, while round objects like oranges were counted in tens.  The use of classifiers obviously marks an intermediary stage between the era when numbers were completely tied to objects and the era when they became contextless as now. We still retain words like ‘twin’ and ‘duet’ to emphasize special cases of ‘twoness’, note also ‘sextet’, ‘octet’ &c.  The complete dissociation of verbal and written numerals from shape and substance is today universally seen as ‘a good thing’ especially by mathematicians. But classifiers were doubtless once extremely useful because they emphasized what people at the time felt to be important about certain everyday objects and activities, and they remain both a picturesque reminder of the origins of mathematics in the world of objects and our sense-perceptions. The removal of all such features from mathematics proper seems to be a necessary evil but at least let us recognize that it is in part an evil : the banning of contextual meaning from mathematics, the language of science and administration, is typical of the depoeticization of the modern world. In particular, there was strong cultural resistance to using the same set of words or sounds for divinities as for people, or for dead and alive people, for grains of corn and beetles. The Mayas found it necessary to have three sets of numerals where the first two, dealing principally with periods of time, were used exclusively by priests and only the third set was used by ‘ordinary people’. Likewise, a Vth century B.C. Athenian tribute list uses different symbols for the number 2 when the amount is respectively “2 talents”, “2 staters” or “2 obols” showing that even at this late date numbers were still at least partially tied to particular sets of objects (Menninger, Number Words and Number Symbols p. 268-9).

Note 1   Thus, for example, Japanese has the classifier hon for all cylindrical objects and Chinese t’iao for all elongated ones. The class word is placed between the numeral and its application in much the same way as we say (or used to say) “ten head of cattle”. Turkish has two classifiers ‘human’ and ‘non-human’.
  Tribal languages used many classifiers or other methods of distinguishing between objects being counted. The Ojibway, a tribe from Northern Ontario, “classify objects according to their hardness. flexibility and dimensionality….while there are also classifiers for counting the two most important artefacts made within the traditional economy, the house and the boat. (…) All of these numeral classifiers for concrete objects ensure that, when counting, expression is given to essential aspects of the object counted, especially those that affect the handling of the object” (Denny, Cultural Ecology of Mathematics in Closs, pp. 148-9).          The Nootkans used different terms for counting or speaking of          “a.) people, men, women, children, salmon, tobacco;          b.) anything round in shape such as the moon, clothing (except trousers), birds, vessels &c.          c.) an object containing many things such as a block of matches, a herd of cattle, a bale of blankets &c., and several other classes of things.”          (Folan, Calendrical and Numerical Systems of the Nootka, in Closs, Editor, p. 106)

 

 

 

 

 

 

 

 

 

Euclid’s Method of proving Unique Prime Factorisatioon

December 1, 2013

It is often said that Euclid (who devoted Books VII – XI of his Elements to Number Theory) recognized the importance of Unique Factorization into Primes and established it by a theorem (Proposition 14 of Book IX). This is not quite correct. Modern authors usually present UPF in the following way

THEOREM Any positive integer N can be written as a product of primes in one and only one way barring changes in order. i.e.  N = pa qb rc…..

        But what Euclid establishes by Book IX Proposition 14 — Heath, whose translation I use throughout calls ‘theorems’ ‘propositions’ — is
“If a number be the least that is measured by prime numbers, it will not be measured by any other prime number except those originally measuring it.”

      Now, from this one can, with the help of one or two other theorems, deduce Unique Prime Factorization (UPF), but Euclid does not actually do so. For one thing, Euclid would need to show that every (natural) number can be presented as a product of primes if Proposition 14 is to have a universal application. He goes some way to doing this in Propositions 31 and 32 of Book VII : Any composite number is measured by some prime number” and “Any number either is prime or is measured by some prime number”. But, for some reason, we lack the clinching Proposition, that all numbers can be written as a product of primes and that there is only one way of doing this barring changes in order.

Euclid’s presentation of Number Theory is so idiosyncratic, not to say perverse, that many readers, flipping through the Elements,  do not even realize that he ever dealt with numbers at all. This is because Euclid insists on presenting (whole) numbers as line segments
A ______________       B __________  and not, as one would expect, as collections of discrete elements, e.g. by such sequences as ● ● ● ● ● ● ● ●  or □ □ □ □  It is true that, by presenting numbers as lines Euclid gains generality : we can see in the above that A > B but we are not limited to specific magnitudes. Also, Euclid did not have the … facility which we have.

However, I doubt if this was the real reason. By Euclid’s time geometry had almost entirely ousted arithmetic as the dominant branch of mathematics much in the way that algebra subsequently ousted geometry. Pride of place in the Elements is given to the theory of proportion developed by Eudoxus. In the books devoted to Number Theory Euclid only deals with whole numbers (presented as line segments) and ratios between whole numbers which mimic ratios between sides of triangles and other figures. He does not mention ‘fractions’ as such though Greek housewives and practical people must have been well acquainted with them. Why is this? Partly no doubt because of the influence of Plato who, though not himself a mathematician, was well versed in the higher mathematics of his time and remains one of the most important theorists in the history of mathematics. Plato’s view that the ‘truths of mathematics’ are in some sense independent of human experience, while nonetheless underlying it, is the view held by  many pure mathematicians today. Plato considered mere calculation with numbers to be a lowly activity, the affair of craftsmen and merchants, while geometry was a discipline that ennobled the practitioner by fixing his eye on the eternal. This explains the radical ‘geometrization’ of number that we find in Euclid.

In his Books on Number Theory, it would seem that Euclid was building on a much older arithmetic tradition which not only presented numbers as discrete entities but actually used objects such as pebbles or shells in calculations and formed them into shapes — which is why we still speak of ‘triangular numbers’, ‘square numbers’ and so forth (Note 1). The material of Book VII, the basic Book dealing with Number Theory, looks as if it goes back a very long way indeed and this  is at once an advantage and a drawback.

It is an advantage because Euclid kicks off with an eminently practical procedure (rather than an abstract theorem in our sense), the so-called Euclidian Algorithm, and makes it the foundation of the entire edifice. Most of Euclid’s proofs are by contradiction and thus ‘non-constructive’  but the Euclidian Algorithm not only demonstrates that a ‘least common measure’ of two or more numbers always exists, but actually shows you how to obtain it. Remarkably, the Euclidian Algorithm works perfectly well in any base, or indeed without any base at all — and this suggests that it is a very ancient procedure. It was quite possibly  discovered before written numbers even existed : in effect, it shows you how to group or bag up two different collections of similarly sized  objects without anything being left over, using the largest  possible bag size. Proposition 1 is a special case of this : when the largest bag size possible turns out to be the unit. Such an outcome  situation must have seemed extraordinary to the people who first discovered it, and indeed mankind has ever since been fascinated by ‘prime numbers’ — they were originally called ‘line numbers’ because they could only be laid out in a line or column, never as a rectangle.

However, probably because they are based on an ancient source, Euclid’s presentation in the Books devoted to Number Theory is not  so impeccably logical as in the other Books. Euclid does not introduce any new Axioms in Book VII, the first of the four books dealing with Number Theory, though he does give twenty-two Definitions. He presumably  assumed that the general Axioms, given in Book I, sufficed. In fact, they do not. Operations with or on numbers differ from operations on geometric figures since plane figures and solids do not have ‘factors’ in the way that numbers do. As Heath notes, Euclid does not state as an Axiom that factorisation is transitive (as we would put it), i.e. “If a ∣ & B ∣ C, then a ∣ C”, nor does he prove it as a theorem though he assumes it throughout. The Euclidian Algorithm would not work without this feature and a large number of other Propositions would be defective. Indeed, as Heath specifies, we not only need the above but the Sum and Difference Factorisation Theorems which, in Euclid’s parlance, would be

If A measures B, and also measures C, then A measures the sum of B and C, also the difference of B and C when they are unequal and B is greater than C.

i.e. a ∣ B  & a ∣ C, then a ∣ (B + C), also a ∣ (B ‒ C) when  B > C 

        An even more serious admission, from our point of view, is that Euclid does not explicitly state the Well-Ordering Principle, namely that Every non-increasing sequence of natural numbers has a least member though he assumes it in various propositions. Given the strong anti-infinity bias of Greek thought, Euclid would doubtless have thought it unnecessary.

Euclid proves Proposition 14 (If a number be the least that is measured by prime numbers, it will not be measured by any other prime number except those originally measuring it) in the following way :

“Let N = pqrs… where  p, q, r…. are primes. Suppose a prime u different from primes p, q, r… and which divides N. Then N = u × b.
But if any prime number divides (m × n) and does not divide m, it must divide n [VII. 30].

Now, pôN and p does not divide u since u, p are primes and u ≠p Therefore, p ∣ b. And the same applies to q, r….
        Therefore, pqr… ∣ b                                                       

But this is contrary to the hypothesis, since b < N and N is the smallest number that can be divided by pqr….
        Therefore, N has no prime factors apart from p, q, r…

It should be noted that this is a Proof by Contradiction and that it applies only to the case where p, q, r… are each of them distinct primes.

What Propositions does this proof rely on?

Firstly, on VII. Proposition 30 “If two numbers by multiplying one another make some number, and any prime number measure the product, it will also measure one of the original numbers.”

This is one of the most important theorems in the whole of Number Theory and I call it the Prime Factor Theorem. What applies here is the special case when one at least of the two original numbers is prime — and a different prime from the ‘dividing number’.
But Euclid also needs to prove, or to have proved, that, N really is, in our terms, the Least Common Multiple of p, q , r…. This he has done in Book VII. Propositions 34 and 35  which detail the procedure for finding the Least Common Multiple, first of two numbers (Prop. 35), and secondly of three or more numbers (Prop. 36). As a special case, Euclid shows that the LCM of two numbers a, b that are prime to each other is ab  and that the procedure can be applied as many times as we wish so that the LCM of a,b,c…. where a, b, c are primes is abc… (He is also scrupulous enough to show (Proposition 29) that a prime and any other ‘number it does not measure’ are prime to each other, which makes any two primes ‘prime to each other’.)

Euclid does not generalize Proposition 14 to powers of these primes, i.e. to our pa qb rc…  though this extension is in effect covered by the propositions about Least Common Multiples VII. 34, 35 and 36 taken together with VII. 31 and 32.
The propositions concerning LCMs are very much what one would expect and are easily assented to. The same does not apply to the Prime Factor Theorem which is by no means ‘intuitively obvious’ nor that easy to establish.

In modern terms Euclid’s proof of the Prime Side Theorem is as follows:

“Suppose p ∣ N (= ab) where p is prime, and p does not divide a.
Then (p, a) = 1  [VII. 29]
Let ab = pm = N where m is some number.
Then p ∣ a = b ∣ m [VII. 19]
But since (p, a) = 1, p/a is in its lowest terms. Therefore m must be a multiple of a and b a multiple of p [VII. 20, 21].
So, if p ∣ ab where p is prime, then either p ∣ a or p ∣ b (or both).”

         The key proposition here is VII. 19, the Cross Ratio Theorem: “If four numbers be proportional, the number produced from the first and fourth will be equal to the number produced from the second and third; and, if the number produced from the first and fourth be equal to that produced from the second and third, the four numbers will be proportional.”   

This cumbersome statement shows the importance of algebraic notation which the Greeks did not have. Remember that Euclid is speaking only of ratios between hypothetical line segments, not of ‘rational numbers’ as modern mathematicians understand them. However, bearing this in mind, Euclid’s proof may be presented thus :

“Let ac/ad  =  c/d = a/b
        But  a/b = ac/bc
So ac/ad  =  ac/bc
This can only be true if ad = bc   

Conversely, let ad = bc
Then ac/ad = ac/bc
However, ac/ad = c/d
Also ac/bc = a/b
Therefore a/b = c/d

The above itself depends on the legitimacy of ‘cancelling out’, likewise the legitimacy of multiplying and dividing numerator and denominator by the same factor. Euclid has already dealt with such issues and I will not trace the derivation any further back. He has, I think, made a proposition by no means obvious — the ‘Prime Factor Theorem’ — entirely acceptable and, if we accept the latter, we must accept Book IX Proposition 14. Apart from some tidying up and expansion, Unique Prime Factorization in the Natural Numbers has been established.
There was in fact another way to prove the Prime Factor Theorem which is more in keeping with the general plan of Book VII since the above theorem can be made to depend on the Euclidian Algorithm. The latter, for those who are not familiar with it or need a reminder, is a foolproof method for finding the lowest common factor of two numbers. Algebraically, take two different whole numbers M and N where M > N 

 M = a N + R1
N = b P + R2
P = c Q + R3
…….

         This sequence will eventually come to an end since the divisors are diminishing i.e. Q < P < N < M
        (This is an example of Euclid appealing to the Well-Ordering Principle without stating it as such.)

        Suppose, the very next line leaves no remainder.
Q = d T
For  N = b P + R= b (cQ + R3) + R2     
N = b P + R= b (cQ + R3) + R2     

 Note 1 :  “It seems clear that the oldest Pythagoreans were acquainted with the formation of triangular and square numbers by means of pebbles or dots; and we judge from the account in Speusippus’s book On the Pythagorean Numbers, which was based on the works of Philolaus, that the latter dealt with linear numbers, polygonal numbers, and plane, and solid numbers of all sorts….”   (Heath, History of Greek Mathematics p. 76)