Pythagorean Theorem

Lagrange's four-square theorem - Wikipedia

数论代考, 四平方和定理The four square theorem 代写

Euler's four-square identity

Jacobi's four-square theorem

PDF] New parametrization of $A^2+B^2+C^2=3D^2$ and Lagrange's four-square theorem

Chapter 3 Greek Number Theory The Role of Number Theory Polygonal, Prime and Perfect Numbers The Euclidean Algorithm Pell's Equation The Chord and Tangent. - ppt download

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

Sum of two squares theorem - Wikipedia

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

An Independent Quaternion Proof of Jacobi's Four Square Theorem