Jacobi's four-square theorem - Wikipedia

Lagrange's four-square theorem - Wikipedia

A Hamilton–Jacobi-based proximal operator

Jacobi's four-square theorem - Wikipedia

History of Dynamics, Galileo Unbound

What is the importance of proofs in mathematics? Do they show the validity of theorems, or are they just as important as the theorems themselves? - Quora

Number Theory and Pi

Lagrange's four-square theorem Wikipedia audio article

File:Jacobi theorem.svg - Wikimedia Commons

Theta function - Wikipedia