Lagrange's three square theorem

Explain Why No Number From This List Is The Sum Of Three Squares: 7, 15, 23, 31, 39, 47, 55, …, by Sundip Tailor, Math Simplified

Jeffrey Aronson: When I Use a Word . . . Power - The BMJ

Full article: Refinements of Lagrange's Four-Square Theorem

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

algorithms - Listing integers as the sum of three squares $m = x^2 + y^2 + z^2$ - Computer Science Stack Exchange

Fermat's Polygonal Number Theorem -- from Wolfram MathWorld

Sum of Squares Theorems Brilliant Math & Science Wiki

The Fundamental Theorem of Arithmetic

Waring's Problem -- from Wolfram MathWorld

Cosets and Lagrange's Theorem. Understanding a fundamental (and…, by Ujjwal Singh

Rolle's theorem - Statement, Proof, Examples, Interpretation

Rolle's Theorem Lagrange's Mean Value Theorem and Examples