Why was this visual proof missed for 400 years? (Fermat's two square theorem)

LAGRANGE'S FOUR SQUARE THEOREM Theorems, Lagrange theorem, Number theory

Chit-chat on Lagrange's four-square theorem

PDF) Lagrange's Four-Square Theorem

PDF) Two refinements of Lagrange's four-square theorem

Table 3 from Extended Lagrange's four-square theorem

number theory - The Four Square Theorem and Integral Apollonian Circle Packings, is there any connection? - Mathematics Stack Exchange

CodeChef - Lagrange's four-square theorem, also known as Bachet's conjecture, states that every natural number can be represented as the sum of four integer squares. This theorem was proven by Joseph Louis

Solved 4. [12 marks] The Modular Operation x mod m = r

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

Euler's four-square identity - Wikipedia