Jacobi s four squares identity Martin Klazar

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1 Jacobi s four squares identity Martin Klazar (lecture on the 7-th PhD conference) Ostrava, September 10, 013 C. Jacobi [] in 189 proved that for any integer n 1, r (n) = #{(x 1, x, x 3, x ) Z ( i=1 x i = n} = 8 d ) d. d n In other words, the number of ways to express n as a sum of four squares of integers equals eight times the sum of the divisors of n not divisible by four. Thus r (n) = 8(1 +...) 8 for every n 1, which gives as a corollary Lagrange s theorem from 1770 that every natural number is a sum of four squares. We give a complete and purely formal proof of Jacobi s identity by generating functions; the proof is due to Hirschhorn [1] in Idea: since r g (n), the number of expressions of an integer n 0 as a sum of g squares of integers, equals to the coefficient of x n in (the formal power series expansion of) ( + x n) g, n= and d n d equals to the coefficient of xn in the Lambert series k, nx kn = nx n 1 x n, d n it suffices to derive the identity ( x n ) = ( nx n 1 x nx n ) n 1 x n (1) Jacobi s identity in the GF language. (We write briefly for + n= and for.) We achieve it by a succession of the next seven identities. 1

2 1. ( 1) n x n = 1 x n 1 + x n.. JTI: (1 x n )(1 + zx n 1 )(1 + z 1 x n 1 ) = z n x n. 3. (z z 1 ) (1 x n )(1 z x n )(1 z x n ) = ( 1) n z n+1 x n(n+1)/.. (1 x n ) 3 = 1 (n + 1)( 1) n x n(n+1)/. 5. (1 x n ) 6 = 1 ((r + 1) (s) )x r +r+s. r,s 6. (1 x n ) 6 = Q n [ 1 8 ( (n 1)x n x n 1 nxn 1 + x n )], where Q n = (1 x n ) (1 + x n ) (1 + x n 1 ). 7. (1 + x n ) (1 x n ) = Q n. The key identity, from which we derive everything, is, the celebrated Jacobi s triple product identity (JTI for short), obtained by C. Jacobi in []. We first deduce (1) from the identities, then derive the seven identities assuming the JTI, and at the end we give a proof for the JTI. We have that ( ( 1) n x n) (id.1) = ( 1 x n ) (the left side of id. 6) = 1 + x n (the left side of id. 7) equals to the ratio of the right sides 1 8 ( (n 1)x n 1 ) nxn. 1 + x n x n

3 Replacing x with x, we obtain the identity ( x n ) = ( (n 1)x n 1 ) + nxn, 1 x n x n which is similar to (1). We rewrite the last sum as ( (n 1)x n 1 ) + nxn 1 x n 1 1 x n ( nx n ) nxn. 1 xn 1 + x n The first sum equals nx n by regrouping, and the summand in the 1 x n second sum equals, by subtracting the two fractions, to nxn. Thus the 1 x n obtained identity is not just similar to but indeed identical to (1). We start from identity, the JTI, and derive from it the rest. 1. We set z = 1 in the JTI and get that ( 1) n x n equals to (1 x n )(1 x n 1 ). Since (1 x n )(1 x n 1 ) = (1 x n ) and (1 x n ) = (1 x n )/(1 + x n ), regrouping yields the right side of We set z = xz in the JTI, then x = x 1/ and multiply the result by z. 3. We differentiate 3 by z, set z = 1 and multiply the result by Squaring gives (1 x n ) 6 = 1 (m + 1)(n + 1)( 1) m+n x (m +m+n +n)/. m,n We split the sum in two subsums, the first with even m + n and the second with odd m + n. In the first subsum we change the variables to r = (m + n)/, s = (m n)/, and in the second to r = (m n 1)/, s = (m+n+1)/. It is easy to check that in both cases m +m+n +n turns into (r +r +s ) and ( 1) m+n (m + 1)(n + 1) into (r + 1) (s). Hence the two subsums coincide and the sign disappears; we get the right side of 5. 5, 6. We write the right side of 5 as a difference of two sums, by the difference (r + 1) (s), and separate in each sum the variables r and s. Then we express the coefficients by differentiation and obtain 1 ( s x s (1 + x d dx ) r x r +r r 3 x r +r x d dx x s). s

4 We replace each of the four s by a using the JTI, setting z = 1 in s and z = x in r ( 1 gets cancelled by 1 + x0 ): (1 x n )(1 + x n 1 ) (1 + x d dx ) (1 x n )(1 + x n ) minus (1 x n )(1 + x n ) x d dx (1 x n )(1 + x n 1 ). We differentiate the infinite products by means of the identity ( f n ) = fn f n/f n and, denoting a = 1 + x n, b = 1 + x n 1, c = 1 x n and taking out the factor Q n = (abc), get the expression Qn [ 1 + x ( (a c) a c (b c) )]. b c The last summand simplifies after an easy calculation to (a /a b /b). We get the right side of We have Qn = (1 x n ) (1 + x n ) (1 + x n 1 ). Since 1 x n = (1 x n )(1 + x n ) and (1 + x n )(1 + x n 1 ) = (1 + x n ), regrouping gives the left side of 7. It remains to prove the JTI (1 x n )(1 + zx n 1 )(1 + z 1 x n 1 ) = z n x n. We give a purely formal proof. Let S(z, x) be the product on the left side. It follows that zx S(zx, x) = S(z, x), because the substitution z zx almost preserves the two arithmetic progressions of odd positive integers in the two exponents of x: it just adds to S(z, x) the factor (1 + z 1 x 1 )/(1 + zx) = 1/zx. Therefore if we expand S(z, x) in the integral powers of z as S(z, x) = a n z n = a n (x)z n (n runs through the whole Z) with the power series coefficients a n C[[x]], comparison of the coefficients of z n on both sides of the functional equation gives the relation x n 1 a n 1 = a n, n Z.

5 Thus a 1 = xa 0 = a 1 (n = 1, 0), a = x 3 a 1 = x 3 a 1 = a (n =, 1) and a = x a 0 = a. In general, a n = a n = x n a 0, n = 1,,..., since (n 1) = n. (The equality a n = a n is immediate also from the symmetry S(z, x) = S(z 1, x).) So we have deduced that S(z, x) = (1 x n )(1 + zx n 1 )(1 + z 1 x n 1 ) = a 0 (x) z n x n and are almost done it only remains to be shown that a 0 = 1. This we prove by three specializations of the last identity (the JTI with the undetermined coefficient a 0 (x)): (i) z = x = 0, (ii) z = i and (iii) z = 1, x = x. Then (i) gives that a 0 (0) = 1, (ii) gives that (1 x n )(1 + x n ) = a 0 (x) ( 1) n x (n) and (iii) gives that (1 x 8n )(1 x 8n ) = a 0 (x ) ( 1) n x n. Clearly, the sums on the right sides are equal. So are the products on the left sides, despite appearance: since (1 x 8n )(1 x 8n ) = (1 x n ), 1 x 8n = (1 x n )(1 + x n ) and (1 x n )(1 x n ) = (1 x n ), the product of (iii) equals to that of (ii). Hence the remaining factors have to be equal too, a 0 (x) = a 0 (x ), which forces that a 0 (x) = a 0 (0) = 1, as we needed to show. This completes the proof of the JTI and of Jacobi s four squares identity. Exercise. Justify rigorously formal manipulations in the above proof. In particular, clarify in what sense does the formal equality (1 x n )(1 + zx n 1 )(1 + z 1 x n 1 ) = a n (x)z n hold and why it is preserved by the substitution z zx. References [1] M. D. Hirschhorn, A simple proof of Jacobi s four-square theorem, Proc. Amer. Math. Soc. 101 (1987), [] C. G. J. Jacobi, Fundamenta nova theoriae functionum ellipticarum, Sumtibus fratrum,

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