Sum of two squares theorem - Wikipedia

PDF) Lagrange's Four-Square Theorem

Lagrange's four-square theorem

Table 3 from Extended Lagrange's four-square theorem

Triangle with Squares 6: Four squares. Level: High School, SAT Prep, College geometry Antonio Gutierrez

Jacobi's four-square theorem

PDF) Algebraic Consequences of Jacobi's Two- and Four-Square Theorems

Pythagorean Theorem

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

Theorems Rectangle Rhombus Square (examples, solutions, worksheets, videos, games, activities)

Waring's Problem -- from Wolfram MathWorld