Sum of two squares theorem - Wikipedia

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

Lagrange's four-square theorem

PDF) A Simple Proof of Jacobi's Four-Square Theorem

Lagrange's Four-Square Theorem -- from Wolfram MathWorld

Discrete quantum computation and Lagrange's four-square theorem

Pythagorean Theorem

Lagrange's Four-Square Theorem by Julian TSE

Stable/Patient Sort: Bit-Manipulation-To-Solve-Problems-Easily-And-Efficiently, PDF, Square Root

PDF) Sums of four squares and Waring's Problem

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