Every non-negative integer is a sum of four integer squares. Lagrange 1770 proof uses Euler's four-square identity (the product of two sums-of-4-squares is itself a sum-of-4-squares) and reduces to primes via quaternions. Complements…
Every non-negative integer is a sum of four integer squares. Lagrange 1770 proof uses Euler's four-square identity (the product of two sums-of-4-squares is itself a sum-of-4-squares) and reduces to primes via quaternions. Complements…