Anonymous ID: 5f9eda April 21, 2024, 7:03 a.m. No.20756112   🗄️.is 🔗kun   >>6119 >>6222

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LAGRANGE’S FOUR SQUARE THEOREM

Euler’s four squares identity. For any numbers a, b, c, d, w, x, y, z

(a2 + b2 + c2 + d2)(w2 + x2 + y2 + z2) = (aw − bx − cy − dz)2+

(ax + bw + cz − dy)2 + (ay + cw + dx − bz)2 + (az + dw + by − cx)2.

Lagrange’s Theorem. Every natural number is the sum of four squares.

Proof. In view of Euler’s identity and 12 + 12 = 2, it suffices to prove that every odd

prime is such a sum.

Lemma 1. If n is even and is a sum of four squares, then so is n

2 .

Proof of Lemma 1. When n = a2 + b2 + c2 + d2 is even, an even number of the squares

will be odd. and so the a, b, c, d can be rearranged so that a, b have the same parity and

so do c, d. Thus n

2 = ( a+b

2

)2 + ( a−b

2

)2 + ( c+d

2

)2 + ( c−d

2

)2.

Lemma 2. If p is an odd prime, then there are integers a, b, c, d and an m so that

0 < a2 + b2 + c2 + d2 = mp < p2

2 .

Proof of Lemma 2. The p+1

2 numbers 02, 12 . . . , ( p−1

2

)2 are pairwise incongruent modulo

p. Hence, by a box argument there are u, v such that u2 ≡ −v2 − 1 (mod p) and 0 <

u2 + v2 + 1 ≤ p2−2p+3

2 .

By Lemma 2 there is an integer m with 0 < m < p so that for some a, b, c, d we have

a2 + b2 + c2 + d2 = mp

and we may suppose that m is chosen minimally. Moreover, by Lemma 1 we may suppose

that m is odd. If m = 1, then we are done. Suppose m 1. If m were to divide

each of a, b, c, d, then we would have m|p contradicting m < p. Choose w, x, y, z so

that w ≡ a (mod m), |w| ≤ m−1

2 , x ≡ −b (mod m), |x| ≤ m−1

2 , y ≡ −c (mod m),

|y| ≤ m−1

2 , z ≡ −d (mod m), |z| ≤ m−1

2 , and then not all of w, x, y, z can be 0. Moreover

w2 +x2 +y2 +z2 ≡ 0 (mod m) and so 0 < w2 +x2 +y2 +z2 = mn ≤ 4 ( m−1

2

)2 = (m−1)2.

Thus 0 < n < m. Now aw−bx−cy−dz ≡ a2+b2+c2+d2 ≡ 0 (mod m), ax+bw+cz−dy ≡

−ab + ab − cd + dc ≡ 0 (mod m), ay + cw + dx − bz ≡ −ac + ac − db + db ≡ 0 (mod m),

az + dw + by − cx ≡ −ad + ad − bc + bc ≡ 0 (mod m). By Euler’s identity m2np is the

sum of four squares and each of the squares is divisible by m2. Hence np is the sum of

four squares. But n < m contradicting the minimality of m.