Jacobi's four-square theorem - Wikipedia

Sum of four Squares

Table 1 from Extended Lagrange's four-square theorem

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

Euler's four-square identity

GitHub - HechenHu/Four-Square_Representation: Using Rabin and Shallit's Algorithm to compute the four-square representation of a natural number as stated in Lagrange's Four-square Theorem.

Lagrange's 4 Square Theorem using Python

Discrete quantum computation and Lagrange's four-square theorem

Lagrange's three square theorem

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

Euler Four-Square Identity -- from Wolfram MathWorld