# = 5 2 and = 13 2 and = (1) = 10 2 and = 15 2 and = 25 2

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1 BEGINNING ALGEBRAIC NUMBER THEORY Fermat s Last Theorem is one of the most famous problems in mathematics. Its origin can be traced back to the work of the Greek mathematician Diophantus (third century a.d.?) who wrote a treatise on solving various kinds of equations. One of his problems was to divide a square into two squares or in modern terminology to solve x + y = z. Diophantus Arithmetica was translated into Latin in 161 and hence became available to western European scholars. Pierre de Pierre de Fermat ( ) Fermat a lawyer by profession but nonetheless one of the greatest mathematicians of all time wrote a note in the margin of his copy of the Arithmetica: To divide a cube into two cubes a fourth power into two fourth powers [and so on] is impossible. I have discovered a marvellous proof of this fact but there is not enough space to write it in the margin [of the book]. It is important to remember that both Diophantus and Fermat were talking about solving equations in integers (actually in rational numbers but in this case it comes to the same thing). So Fermat s claim is in effect that if n is an integer greater than then the equation x n +y n = z n has no solutions in which xy and z are positive integers. This statement has become known as Fermat s Last Theorem. Fermat wrote his note in about 1637 but did not leave a proof either in the margin or elsewhere; we can only speculate as to whether or not he actually had a correct proof. Three centuries of work by many mathematicians showed that the problem is actually far harder than (presumably) anyone could have imagined and opened up new areas of mathematics in the process. Probably we must assume that even Fermat got it wrong sometimes! The Last Theorem was finally proved by Andrew Wiles in Andrew Wiles (1953 ) 1 Pythagorean triples. Let s begin with Diophantus problem of solving x +y = z. A solution in positive integers of this equation is known as a Pythagorean triple because of the obvious connection with Pythagoras Theorem. There are many well known solutions for instance = 5 and = 13 and = 17. (1) There is also = 8191 which was known to the Babylonians (ca b.c.) Collecting examples however is rather unsatisfying and we would like if we can to find all Pythagorean triples. This turns out to be possible through the use of a few basic facts about the integers. First let s look at a few more Pythagorean triples. We have for example = 10 and = 15 and = 5 and so on. These are not very interesting however as they are really just disguised versions of = 5. So we ll assume initially that xy and z have no common factor. Then at the end of the problem we can just multiply the common factor back in again should we wish to write down all solutions. So let s try to find solutions of x + y = z in which xy and z are positive integers with no common factor. Looking at the examples (1) you might notice that in each case x is odd and y even or vice versa. Is this always true? In principle there are two other possibilities. Suppose that x and y are both even. Then x + y is even so z is even so z is even. But this means that xy and z all have a common factor of (or perhaps more) and we have agreed to ignore this possibility. So this case can be ruled out. Could x and y both be odd? If so then x and y are odd and so z is even. But now consider the remainder when these squares are divided by 4. We have x = (odd) = (p + 1) = 4p + 4p + 1 = 4(p + p) + 1 and so the remainder is 1; the same goes for y. On the other hand z = (even) = (q) = 4q

2 and the remainder is 0. Altogether if we divide by 4 then the remainder on the left hand side is while on the right hand side it is 0. This is impossible. As we have eliminated all possible alternatives we may conclude that the fact we observed in examples (1) is actually true for all Pythagorean triples with no common factor: one of the numbers x and y is odd the other even. Which is which? since x + y is the same as y + x it doesn t matter. Let s assume that x is odd and y is even; once we have solved to find x and y if we want all solutions we must remember to allow for an interchange of x and y. Before proceeding let s also comment that we now know that z is odd too. What we have done so far really amounts to clearing away some of the minor difficulties. We now turn to the key step which is to rearrange the equation x + y = z and factorise. Using the difference of two squares we find x = z y = (z + y)(z y). () Now what can we deduce from the fact that the product of the numbers z+y and z y is a square? Clearly one possibility is that the two numbers themselves are both squares; however by looking at an example we see that this is not the only possibility. For example 900 is a square; it can be factorised as = = 4 5 = = = and so on. In some cases (enclosed in boxes) each of the two factors is a square in other cases not. However if we look closely at these examples we may notice that when the factors are not squares then they have a common factor thus 3 and 300 are both multiples of 3 while 5 and 180 are both multiples of 5. In fact this is always true; the following result is very important. Theorem. Suppose that p and q are positive integers that they have no common factor and that their product pq is a square. Then both p and q are squares. 3 Returning to our investigation of Pythagorean triples () tells us that z + y and z y will both be squares if we can be sure that they have no common factor. Well suppose that d > 1 and d is a factor of both z + y and z y. First we can say that d is odd; for z is odd and y is even so z + y and z y are odd and odd numbers don t have even factors. Next d is factor of the sum and difference of z + y and z y that is of z and y; and since d is odd it must be a factor of both z and y. But this is impossible: it means that d is also a factor of x and we are back in the case which we excluded a couple of pages back. So now we know that z + y and z y are both squares let s give them some appropriate names say z + y = a and z y = b. Here a and b are positive integers; they are odd because z +y and z y are odd; they have no common factor as z +y and z y have none; and a > b because z + y > z y. Moreover we can now easily find xy and z in terms of a and b. We have z = and (z + y) + (z y) so x = ab. And that s it! = a + b y = x = (z + y)(z y) = a b (z + y) (z y) = a b Theorem. All solutions of x + y = z in which xyz are positive integers with no common factor and x is odd are given by the formulae x = ab y = a b z = a + b where a and b are odd positive integers with no common factor and a > b. Example. Want to find a large Pythagorean triple without trial and error? Take for instance a = and b = 431; use the above formulae (and a calculator!) to get =

3 Exercises. 1. What values of a and b will give the Babylonian triple mentioned on page?. We should have checked that and 431 have no common factor. Confirm this by using the Euclidean algorithm. (Look it up or ask your teacher!) Another example. The ideas used in studying Pythagorean triples can be extended and used to investigate a wide variety of other equations where we seek solutions in positive integers. A particularly interesting and not excessively difficult example is x + = y 3 ; this is sometimes called Mordell s equation after Louis Mordell who made an extensive study of equations having the form x + k = y 3 where k is a specified integer. Louis Mordell ( ) It is not hard by trial and error to find a solution x = 5 y = 3 to Mordell s equation. Once again however we should like to find all solutions; we ll try to do so by taking advantage of the methods we used above. So we wish to find all positive integers x and y which satisfy the equation x + = y 3. (3) Let s jump straight to the key step of factorising part of the equation. This doesn t seems to be as easy as the last time since we can t find a difference of two squares. Or can we? how about this?! ( x + )( x ) = y 3. This might look totally crazy as the factors on the left hand side are not integers (they are not even real numbers!). However they are elements of the set S = { a + b a and b are integers and it is an extraordinary fact that numbers in S behave in many ways like the usual integers. First we have a result which is very similar to that at the bottom of page 3. 5 Theorem. Suppose that p and q are numbers in S that they have no common factor and that their product pq is a cube. Then both p and q are cubes. Before we can use this theorem we need to know a few things about factors in S. For a start what does it even mean to say that p is a factor of q in S? It means that the quotient q/p is in S. For example = 1 + is in S so 3 is a factor of 7 +. On the other hand = is not in S so 3 is not a factor of It s probably not too surprising that the number is a particularly important element of S. We ll show that if x and y form a positive integer solution of (3) then is not a factor of x. For if it were we should have x = ( a + b ) for some a + b in S; taking the complex conjugate of both sides gives x = ( a b ). Multiplying these last two equations we find that x = (a + b ) and so x is even. Hence y is also even; dividing both sides of (3) by 4 leaves a remainder of on the left hand side and 0 on the right hand side which is impossible. Thus is not a factor of x. Note the similarity of this last step to the argument on page. The next step is to check for any common factors of x + and x so that we can apply the theorem at the top of this page. Suppose that a + b is a factor of both these numbers; then it is a 6

4 factor of their difference. (It s a factor of their sum too but this turns out to be unimportant.) That is ( a + b )( c + d ) = for some c + d in S. As in the previous paragraph we can take conjugates and multiply to obtain (a + b )(c + d ) = 8. Now note that a + b and c + d are just positive integers. So this last equation tells us that a + b is a factor (in the ordinary sense!) of 8 and there are just four possibilities: a + b = 1 4 or 8. We could try to eliminate possibilities one by one but there is a neat short cut which will wipe out three cases at a stroke. If a + b is even then a is even say a = e and a + b = ( b e ) ; thus is a factor of x+ and hence is a factor of x; but we know that this is impossible. Therefore a + b cannot be even and we have a + b = 1 so a = 1 and b = 0 so (at last!) a + b = ±1. What we have just shown is that x+ and x have no common factor except 1 (and 1). Since their product is a cube each is itself a cube and so we can write x + = ( a + b ) 3 = (a 3 6ab ) + (3a b b 3 ). Equating real and imaginary parts x = a 3 6ab and 1 = 3a b b 3. 7 We have eliminated all the complex numbers from the problem and have reduced it to two equations in ordinary integers. The second of these gives a factorisation 1 = b(3a b ). Thus b is a factor of 1 that is b = ±1 and in each case we can calculate ax and y. One possibility is b = 1 3a b = 1 and so a = ±1 x = ±5 y = 3; the other b = 1 does not work since (check this for yourself) it does not give integral values for ax and y. If we are looking for positive solutions we must ignore x = 5 and the problem is finished. Theorem. The only solution of x + = y 3 in which x and y are positive integers is x = 5 y = 3. Comment. In other words the equation has no solution except for the one we found by trial and error. This is perhaps a surprising contrast to the case of Pythagorean triples where we found an infinite collection of solutions. Complications. Our determination of all solutions of Mordell s equation depended on the similarity of numbers a + b to the ordinary integers. If we investigate other equations there are a number of difficulties which may arise. Let s look briefly at two of these. The problem of units. The key result we used in both of our previous investigations went something like this: Suppose that p and q are numbers with no common factor and that their product pq is an nth power. Then both p and q are nth powers. As we have seen this is true if the nth powers it refers to are squares of positive integers and also if they are cubes of numbers in S. However in other situations the result is untrue. For example consider all (not only positive) integers. We can write expressions such as 900 = ( 9) ( 100) (4) and clearly the factors are not squares. This is quite easy to fix Suppose that p and q are integers that they have no common factor and that their product pq is a square. Then each of p and q is plus or minus a square 8

5 but sometimes there are further difficulties. For example in the set of numbers T = { a + bi a and b are integers where i = 1 we have (3 + i) = 8 + 6i = (4 + 3i). (5) The left hand equality shows that 8 + 6i is a square; also the factors on the right hand side have no common factor; but these factors are not squares. Exercise. Confirm this by writing = (a + bi) expanding solving to find a and b and showing that they are not integers. For T a correct version of our basic result is as follows Suppose that p and q are in T that they have no common factor and that their product pq is a square. Then each of p and q is u times a square where u is 1 1 i or i. Indeed the problem in example (5) can be resolved by observing that = i(1 i) and 4 + 3i = i(1 + i). Numbers such as 1 1 i and i in T are called units. They are factors of 1 because 1 = (i)( i) and so on and they are basically unimportant in factorisations. Thus for example we can write 1 as 1 = 3 7 = (3i) ( 7i) = ( 3) ( 7) = ( 3i) (7i) and none of these expressions is really a better factorisation than the others. There are even worse problems in the set Here we have U = { a + b 3 a and b are integers. 1 = ( + 3 )( 3 ) = ( )( ) and so all four of the factors shown are units. In fact U has infinitely many units. Exercise. Prove it! Hint. First show that any power of a unit is also a unit. 9 A version of our key result which applies to many further sets of numbers is as follows. Theorem. Suppose that p and q are elements of a set with unique factorisation that they have no common factor and that their product pq is an nth power. Then each of p and q is equal to a unit times an nth power. Exercise. Consider the numbers p = and q = in U. Show that p and q have no common factors (except for units) and that their product is a square but that neither p nor q is a square. Reconcile these facts with the above theorem. The problem of non unique factorisation. To clarify the above theorem we need to explain what is meant by a set with unique factorisation. First consider again the ordinary integers. Any integer (except 0) can be decomposed into a product of prime numbers those which cannot be factorised any further. For example Alternatively we could write 60 = = 3 5 or 60 = ( 3) ( ) 5 ; but changing the order and inserting units are not very important so in a sense these three factorisations are really the same. We shall say that 60 has only one factorisation into primes. As you may already know this is true not only for 60 but for all non zero integers. We do have of course 60 = 4 15 = 6 10 (6) but this is a fake non unique factorisation since both products can be factorised further and each will end up as 3 5. Theorem. The Fundamental Theorem of Arithmetic. Any non zero integer can be factorised into primes in one and only one way. Exercise. What about 1 and 1? 10

6 By a set with unique factorisation we mean one in which such a property holds. Thus the integers have unique factorisation; and it is possible to prove that the sets S T and U considered above also have unique factorisation. It also turns out that the key result we have used in solving Diophantine equations is a consequence of unique factorisation. Unfortunately unique factorisation is by no means guaranteed when we study Diophantine equations by the above methods. If we think about a different case of Mordell s equation x + 5 = y 3 the approach used above would lead us to consider But consider the following: V = { a + b 5 a and b are integers. 6 = 3 = ( )( 1 5 ). (7) If we want to reconcile these two expressions by splitting them into primes we would start by trying to factorise in V. That is we need ( a + b 5 )( c + d 5 ) = ; (8) by the method of taking conjugates and multiplying we obtain (a + 5b )(c + 5d ) = 4. Thus a + 5b is a factor of 4. It is not hard to show that a + 5b = is impossible; if a + 5b = 1 then a = ±1 and b = 0 so a + b 5 = ±1 which is a unit; if a + 5b = 4 then c + 5d = 1 and by the same argument c+d 5 is a unit. Therefore (8) is not a proper factorisation as it holds only when one of the factors is a unit. By similar means we show that 3 and and 1 5 cannot be factorised either. Hence (7) is a genuine example of non unique factorisation and is not at all like the fake example (6). In fact the theorem at the top of page 10 does not apply at all to the set V. Exercise. Show that p = 5 and q = +3 5 have 11 no common factor in V and that pq is a square but that neither p nor q is a square or the negative of a square. The topic of what kinds of numbers have unique factorisation has been the subject of an immense amount of mathematical research. Fermat s Last Theorem. We conclude with one more look at Fermat s Last Theorem. For n = 3 we consider the equation x 3 + y 3 = z 3 ; (9) if you are handy with complex numbers you can factorise this as (x + y)(x + ωy)(x + ω y) = z 3 where ω = cos 3 π + isin 3π. To apply the above methods we need to consider W = {a + bω a and b are integers ; it turns out that W has unique factorisation and the same ideas will suffice to show that (9) has no solution though the details are far from easy. In fact all this will work if the exponent n is from 3 up to ; unfortunately when n = 3 we need to employ a set which turns out not to have unique factorisation and the method collapses. To tell how Fermat s Last Theorem was eventually proved in its entirety would be a different longer and much harder story. Acknowledgement. Portraits of mathematicians in this article were obtained from the History of Mathematics website at the University of St. Andrews Scotland www-history.mcs.st-andrews.ac.uk. A visit to this fascinating site is warmly recommended! David Angell School of Mathematics University of New South Wales Sydney 05 web.maths.unsw.edu.au/~angell 1

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