Waring's Problem -- from Wolfram MathWorld

Jacobi's four-square theorem - Wikipedia

Solved In Number Theory, Lagrange's four-square theorem

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.

Number Theory Sums of Squares Part 7.

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

Sum of two squares theorem - Wikipedia

SOLVED: In the following sequence of problems, we will start the proof of the Four-Square Theorem conjectured in the third century by Diophantus and proven by Lagrange in 1770 (since it took

Sum of squares, Part-2, Sum of four squares

Sum of Squares Function -- from Wolfram MathWorld

SOLUTION: Legendre's two, three and four square theorem (number theory) - Studypool