An Independent Quaternion Proof of Jacobi's Four Square Theorem

Solved In Number Theory, Lagrange's four-square theorem

Triangle with Squares 6: Four squares. Level: High School, SAT Prep, College geometry Antonio Gutierrez

Lagrange's four-square theorem - Wikiwand

Lagrange's Four-Square Theorem Seen Using Polygons and Lines

PDF) Quarternions and the Four Square Theorem

RISIBUES 221 orem 8.35 (Lagrange's Four-Square

Pythagorean Theorem

Proving the Pythagorean Theorem. Some algebraic and geometric proofs of…, by Michele Diodati, Not Zero

Waring's Problem -- from Wolfram MathWorld

Euler Four-Square Identity -- from Wolfram MathWorld