Euler's four-square identity

Lagrange's four-square theorem

Table 3 from Extended Lagrange's four-square theorem

FERMAT'S FOUR SQUARES THEOREM Several years ago, after

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.

Pythagorean Theorem

Lagrange's Four-Square Theorem by Julian TSE

Sum of two squares theorem - Wikipedia

Euler's four-square identity - Wikipedia

Euler Four-Square Identity -- from Wolfram MathWorld

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