Pythagorean triples and sums of squares

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1 Pythagorean triles and sums of squares Robin Chaman 16 January Pythagorean triles A Pythagorean trile (x, y, z) is a trile of ositive integers satisfying z 2 + y 2 = z 2. If g = gcd(x, y, z) then (x/g, y/g, z/g) is also a Pythagorean trile. It follows that if g > 1, (x, y, z) can be obtained from the smaller Pythagorean trile (x/g, y/g, z/g) by multilying each entry by g. It is natural then to focus on Pythagorean triles (x, y, z) with gcd(x, y, z) = 1 these are called rimitive Pythagorean triles. It will be useful to note a basic fact about rimitive Pythagorean triles. Theorem 1 Let (x, y, z) be a rimitive Pythagorean trile. Then gcd(x, y) = gcd(x, z) = gcd(y, z) = 1. Proof Suose gcd(x, y) > 1. Then there is a rime with x and y. Then z 2 = x 2 + y 2 0 (mod ). As z 2 then z and so gcd(x, y, z), contradicting (x, y, z) being a rimitive Pythagorean trile. Thus gcd(x, y) = 1. The roofs that gcd(x, z) = 1 and gcd(y, z) = 1 are similar. Considering things modulo 4 we can determine the arities of the numbers in a rimitive Pythagorean trile. Theorem 2 If (x, y, z) is a rimitive Pythagorean trile, then one of x and y is even, and the other odd. (Equivalently, x + y is odd.) Also z is odd. Proof Note that if x is even then x 2 0 (mod 4) and if x is odd then x 2 1 (mod 4). If x and y are both odd then x 2 y 2 1 (mod 4). Hence z 2 x 2 + y 2 2 (mod 4), which is imossible. If x and y are both even, then gcd(x, y) 2 contradicting Theorem 1. We conclude that one of x and y is even, and the other is odd. In any case, z z 2 = x 2 + y 2 x + y (mod 2), so z is odd. 1

2 As the rôles of x and y in Pythagorean triles are symmetric, it makes little loss in generality in studying only rimitive Pythagorean triles with x odd and y even. We can now rove a theorem characterizing rimitive Pythagorean triles Theorem Let (x, y, z) be a rimitive Pythagorean trile with x odd. Then there are r, s N with r > s, gcd(r, s) = 1 and r + s odd, such that x = r 2 s 2, y = 2rs and z = r 2 + s 2. Conversely, if r, s N with r > s, gcd(r, s) = 1 and r + s odd, then (r 2 s 2, 2rs, r 2 + s 2 ) is a rimitive Pythagorean trile. Proof Let (x, y, z) be a rimitive Pythagorean trile with x odd. Then y is even and z is odd. Let a = 1(z x), b = 1 (z + x) and c = y/2. Then a, b, 2 2 c N. Also (z x)(z + x) ab = = z2 x 2 = y = c2. Let g = gcd(a, b). Then g (a + b) and g (b a); that is g z and g x. As gcd(x, z), by Theorem 1, then g = 1, that is gcd(a, b) = 1. Let be a rime factor of a. Then b, so v (b) = 0. Hence v (a) = v (a) + v (b) = v (ab) = v (c 2 ) = 2v (c) is even. Thus a is a square. Similarly b is a square. Write a = s 2 and b = r 2 where r, s N. Then gcd(r, s) a and gcd(r, s) b; as a and b are corime, gcd(r, s) = 1. Now x = b a = r 2 s 2 ; therefore r > s. Also z = a + b = r 2 + s 2. As c 2 = ab = r 2 s 2, c = rs and so y = 2rs. Finally as x is odd, then 1 x = r 2 + s 2 r + s; that is r + s is odd. This roves the first half of the theorem. To rove the second art, let r, s N with r > s, gcd(r, s) = 1 and r + s odd. Set x = r 2 s 2, y = 2rs and z = r 2 + s 2. Certainly y, z N and also x N as r > s > 0. Also x 2 +y 2 = (r 2 s 2 ) 2 +(2rs) 2 = (r 4 2r 2 s 2 +s 4 )+4r 2 s 2 = r 4 +2r 2 s 2 +s 4 = z 2. Hence (x, y, z) is a Pythagorean trile. Certainly y is even, and x = r 2 s 2 r s r + s (mod 2): x is odd. To show that (x, y, z) is a rimitive Pythagorean trile we examine g = gcd(x, z). As x is odd, g is odd. Also g (x 2 + z 2 ) and g (z 2 x 2 ), that is g 2s 2 and g 2r 2. As r and s are corime, then gcd(2r 2, 2s 2 ) = 2, and so g 2. As g is odd g = 1. Hence (x, y, z) is a rimitive Pythagorean trile. We now aly this to the roof of Fermat s last theorem for exonent 4. 2

3 Theorem 4 There do not exist x, y, z N with x 4 + y 4 = z 4. (1) Proof In fact we rove a stronger result. We claim that there are no x, y, u N with x 4 + y 4 = u 2. (2) A natural number solution (x, y, z) to (1) gives one for (2), namely (x, y, u) = (x, y, z 2 ). Thus it suffices to rove that (2) is insoluble over N. We use Fermat s method of descent. Given a solution (x, y, u) of (2) we roduce another solution (x, y, u ) with u < u. This is a contradiction if we start with the solution of (2) minimizing u. Let (x, y, u) be a solution of (2) over N with minimum ossible u. We claim first that gcd(x, y) = 1. If not, then x and y for some rime. Then 4 (x 4 + y 4 ), that is, 4 u 2. Hence 2 u. Then (x, y, u ) = (x/, y/, u/ 2 ) is a solution of (2) in N with u < u. This is a contradiction. Hence gcd(x, y) = 1. As gcd(x, y) = 1 then gcd(x 2, y 2 ) = 1, and so (x 2, y 2, u) is a rimitive Pythagorean trile by (2). By the symmetry of x and y we may assume that x 2 is odd and y 2 is even, that is, x is odd and y is even. Hence there are r, s N with gcd(r, s) = 1 x 2 = r 2 s 2, y 2 = 2rs, u = r 2 + s 2. Then x 2 +s 2 = r 2, and as gcd(r, s) = 1 then (x, s, r) is a rimitive Pythagorean trile. As x is odd, there exist a, b N with gcd(a, b) = 1 and x = a 2 b 2, s = 2ab, r = a 2 + b 2. Then y 2 = 2rs = 4(a 2 + b 2 )ab, equivalently (y/2) 2 = ab(a 2 + b 2 ) = abr. (Recall that y is even.) If is rime and gcd(a, r) then b 2 = (a 2 + b 2 ) a 2 0 (mod ) and so b. This is imossible, as gcd(a, b) = 1. Thus gcd(a, r) = 1. Similarly gcd(b, r) = 1. Now abr is a square. If a, then b and r. Thus v (a) = v (abr)

4 is even, and so a is a square. Similarly b and r are squares. Write a = x 2, b = y 2 and r = u 2 where x, y, u N. Then so (x, y, u ) is a solution of (2). Also u 2 = a 2 + b 2 = x 4 + y 4 u u 2 = a 2 + b 2 = r r 2 < r 2 + s 2 = u. This contradicts the minimality of u in the solution (x, y, u) of (2). Hence (2) is insoluble over N. Consequently (1) is insoluble over N. 2 Sums of squares For k N we let S k = {a a 2 k : a 1,..., a k Z} be the set of sums of k squares. Note that we allow zero; for instance 1 = S 2. The sets S 2 and S 4 are closed under multilication. Theorem 5 1. If m, n S 2 then mn S If m, n S 4 then mn S 4. Proof Let m, m S 2. Then m = a 2 + b 2 and n = r 2 + s 2 where a, b, r, s Z. By the two-square formula, (a 2 + b 2 )(r 2 + s 2 ) = (ar bs) 2 + (as + br) 2, it is immediate that mn S 2. Let m, m S 4. Then m = a 2 + b 2 + c 2 + d 2 and n = r 2 + s 2 + t 2 + u 2 where a, b, c, d, r, s, t, u Z. By the four-square formula, (a 2 + b 2 + c 2 + d 2 )(r 2 + s 2 + t 2 + u 2 ) = (ar bs ct du) 2 + (as + br + cu dt) 2 it is immediate that mn S 4. + (at bu + cr + ds) 2 + (au + bt cs + dr) 2, We remark that the two-square theorem comes from comlex numbers: (a 2 + b 2 )(c 2 + d 2 ) = a + bi 2 c + di 2 = (a + bi)(c + di) 2 = (ac bd) + (ad + bc) 2 = (ac bd) 2 + (ad + bc) 2. 4

5 Similarly the four-square theorem comes from the theory of quaternions (if you know what they are). We can restrict the ossible factorizations of a sum of two squares. Recall that if is rime, and n is an integer, then v (n) denotes the exonent of the largest ower of dividing n: v(n) n but v(n)+1 n. Theorem 6 Let be a rime with (mod 4) and let n N. If n S 2 then v (n) is even. Proof Let n = a 2 + b 2 with a, b Z and suose n. We aim to show that a and b. Suose a. Then there is c Z with ac 1 (mod( ). ) Then 0 c 2 n = (ac) 2 + (bc) ( 2 ) 1 + (bc) 2 (mod ). This imlies that = 1, but we know that = 1 when (mod 4). This contradiction roves that a. Similarly b. Thus 2 (a 2 + b 2 ) = n and n/ 2 = (a/) 2 + (b/) 2 S 2. Let n S 2 and k = v (n). We have seen that if k > 0 then k 2 and n/ 2 S 2. Note that v (n/ 2 ) = k 2. Similarly if k 2 > 0 (that is if k > 2) then k 2 2 (that is k 4) and n/ 4 S 2. Iterating this argument, we find that if k = 2r + 1 is odd, then n/ 2r S 2 and v(n/ 2r ) = 1, which is imossible. We conclude that k is even. If n N, we can write n = rm 2 where m 2 is the largest square dividing n and r is squarefree, that is either r = 1 or r is a roduct of distinct rimes. If any rime factor of r is congruent to modulo 4 then v (n) = 1+2v (m) is odd, and n / S 2. Hence, if n S 2, the only ossible rime factors of r are = 2 and the congruent to 1 modulo 4. Obviously 2 = S 2. It would be nice if all rimes congruent to 1 modulo 4 were also in S 2. Fortunately, this is the case. Theorem 7 Let be a rime with 1 (mod 4). Then S 2. ( ) Proof As 1 (mod 4) then = 1 and so there is u Z with u 2 (mod ). Let A = {(m 1, m 2 ) : m 1, m 2 Z, 0 m 1, m 2 < }. Then A has (1 + s) 2 elements, where s is the integer art of, that is, s < s + 1. Hence A >. For m = (m 1, m 2 ) R 2 define φ(m) = um 1 + m 2. Then φ is a linear ma from R 2 to R, and if m Z 2 then φ(m) Z. As A >, the φ(m) for m A can t all be distinct modulo. Hence there are distinct m, n A with φ(m) φ(n) (mod ). Let a = m n. Then 5

6 φ(a) = φ(m) φ(n) 0 (mod ). Let a = (a, b). Then a = m 1 n 1 where 0 m 1, n 1 < so that a <. Similarly b <. Then a 2 + b 2 < 2. As m n then a (0, 0) and so a 2 +b 2 > 0. But 0 φ(a) = ua+b (mod ). Hence b ua (mod ) and so a 2 + b 2 a 2 + ( ua) 2 a 2 (1 + u 2 ) 0 (mod ). As a 2 + b 2 is a multile of, and 0 < a 2 + b 2 < 2, then a 2 + b 2 =. We conclude that S 2. We can now characterize the elements of S 2. Theorem 8 (Two-square theorem) Let n N. Then n S 2 if and only if v (n) is even whenever is a rime congruent to modulo 4. Proof If n S 2, is rime and (mod 4) then v (n) is even by Theorem 7. If v (n) is even whenever is a rime congruent to modulo 4 then = rm 2 where each rime factor of r is either 2 or congruent to 1 modulo 4. By Theorem 7 all such lie in S 2. Hence By Theorem 5 r S 2. Hence r = a 2 + b 2 where a, b Z and so n = rm 2 = (am) 2 + (bm) 2 S 2. The reresentation of a rime as a sum of two squares is essentially unique. Theorem 9 Let be a rime. If = a 2 + b 2 = c 2 + d 2 with a, b, c, d N then either a = c and b = d or a = d and b = c. Proof Consider (ac + bd)(ad + bc) = a 2 cd + abc 2 + abd 2 + b 2 cd = (a 2 + b 2 )cd + ab(c 2 + d 2 ) = cd + ab = (ab + cd). As (ac + bd)(ad + bc) then either (ac + bd) or (ad + bc). Assume the former the latter case can be treated by reversing the rôles of c and d. Now ac + bd > 0 so that ac + bd. Also (ac + bd) 2 + (ad bc) 2 = a 2 c 2 + 2abcd + b 2 d 2 + a 2 d 2 2abcd + b 2 c 2 = a 2 c 2 + b 2 d 2 + a 2 d 2 + b 2 c 2 = (a 2 + b 2 )(c 2 + d 2 ) = 2. As ac + bd, the only way this is ossible is if ac + bd = and ad bc = 0. Then ac 2 + bcd = c and ad 2 bcd = 0, so adding gives a(c 2 + d 2 ) = c, that is a = c, so that a = c. Then c 2 + bd = = c 2 + d 2 so that bd = d 2, so that b = d. We wish to rove the theorem of Lagrange to the effect that all natural numbers are sums of four squares. It is crucial to establish this for rimes. 6

7 Theorem 10 Let be a rime. Then S 4. Proof If 1 (mod 4) then there are a, b Z with = a 2 + b (Theorem 7) so that S 4. Also 2 = S 4 and = S 4. We may assume that > and that (mod 4). ( ) As a consequence =. ( ) ( ) w w Let w be the smallest ositive integer with =. Then = 1 ( ) ( ) ( ) and = = 1. Hence there are u, v Z with w 1 u 2 w w (mod ) and w v 2 (mod ). Then 1 + u 2 + v (w 1) w 0 (mod ). Let B = {(m 1, m 2, m, m 4 ) : m 1,..., m 4 Z, 0 m 1,..., m 4 < }. Then B has (1 + s) 4 elements, where s is the integer art of, that is, s < s + 1. Hence A > 2. For m = (m 1, nm 2, m, m 4 ) define ψ(m) = (um 1 +vm 2 +m, vm 1 +um 2 +m 4 ). Then ψ is a linear ma from R 4 to R 2. If m Z 4 then ψ(m) Z 2. We write (a, b) (a, b ) (mod ) if a a (mod ) and b b (mod ). If we have a list (a 1, b 1 ),..., (a N, b N ) of vectors in Z 2 with N > 2, then there must be some i and j with (a i, b i ) (a j, b j ) (mod ). This haens for the vectors ψ(m) with m B as B > 2. There are distinct m, n B with ψ(m) ψ(n) (mod ). Let a = m n. Then ψ(a) = ψ(m) ψ(n) 0 (mod ). Let a = (a, b, c, d). Then a = m 1 n 1 where 0 m 1, n 1 < so that a <. Similarly b, c, d <. Then a 2 +b 2 +c 2 +d 2 < 4. As m n then a (0, 0, 0, 0) and so a 2 +b 2 +c 2 +d 2 > 0. Now (0, 0) φ(a) = (ua + vb + c, va + ub + d) (mod ). Hence c ua vb (mod ) and d va ub (mod ). Then a 2 + b 2 + c 2 + d 2 a 2 + b 2 + (ua + vb) 2 + (va ub) 2 = (1 + u 2 + v 2 )(a 2 + b 2 ) 0 (mod ) As a 2 + b 2 + c 2 + d 2 is a multile of, and 0 < a 2 + b 2 + c 2 + d 2 < 4, then a 2 + b 2 + c 2 + d 2 {, 2, }. When a 2 + b 2 + c 2 + d 2 = then certainly S 4. Alas, we need to consider the bothersome cases where a 2 + b 2 + c 2 + d 2 = 2 or. Suose that a 2 + b 2 + c 2 + d 2 = 2. Then a 2 + b 2 + c 2 + d 2 2 (mod 4) so that two of a, b, c, d are odd and the other two even. Without loss of generality a and b are odd and c and d are even. Then 1(a + b), 1 (a b), (c + d) and 1 (c d) are all integers, and a simle comutation gives 2 2 ( ) 2 ( ) 2 ( ) 2 ( ) 2 a + b a b c + d c d = a2 + b 2 + c 2 + d 2 =

8 so that S 4. Finally suose that a 2 + b 2 + c 2 + d 2 =. Then a 2 + b 2 + c 2 + d 2 is a multile of but not 9. As a 2 0 or 1 (mod ) then either exactly one or all four of a, b, c and d are multiles of. But the latter case is imossible (for then a 2 + b 2 + c 2 + d 2 would be a multile of 9), so without loss of generality a and b, c, d ±1 (mod ). By relacing b by b etc., if necessary, we may assume that b c d 1 (mod ). Then Then 1(b+c+d), 1(a+b c), 1 (a + c d), 1 (a + d b), are all integers, and a simle comutation gives ( ) 2 ( ) 2 ( ) 2 ( ) 2 b + c + d a + b c a + c d a + d b = a2 + b 2 + c 2 + d 2 = so that S 4. We can now rove Lagrange s four-square theorem. Theorem 11 (Lagrange) If n N then n S 4. Proof Either n = 1 = S 4, or n is a roduct of a sequence of rimes. By Theorem 10, each rime factor of n lies in S 4. Then by Theorem 5, n S 4. We finish with some remarks about sums of three squares. This is a much harder toic than sums of two and of four squares. One reason for this is that the analogue of Theorem 5 is false. Let m = = and n = 5 = Then m S and n S but mn = 15 / S. It follows that we cannot reduce the study of sums of three squares to this roblem for rimes. Theorem If m S then m 7 (mod 8). 2. If 4n S then n S. Proof Let m = a a a 2. As a 2 j 0 or 1 (mod 4) then m k (mod 4) where k is the number of odd a j. If m 7 (mod 8) then m (mod 4) and so all of the a j are odd. But if a j is odd, then a 2 j 1 (mod 8) and so m = a a a 2 (mod 8), a contradiction. Let m = 4n = a a a 2. As m 0 (mod 4) then all of the a j are even. Hence n = (a 1 /2) 2 + (a 2 /2) 2 + (a /2) 2 S. As a consequence, if n = 4 k m where k is a nonnegative integer and m 7 (mod 8) then n S. Gauss roved in his Disquisitiones Arithmeticae that if n N is not of this form, then n S. Alas, all known roofs are too difficult to be resented in this course. 8