Lagrange's Four Square Theorem

Theorem
Every positive integer can be expressed as a sum of four squares.

Also known as
Some sources use the plural form, as Lagrange's four squares theorem.

Some omit Lagrange's name, and refer to this as just the four squares theorem.

Some sources refer to this as just Lagrange's theorem, but this is ambiguous as there is more than one theorem with Lagrange's name on it.

Also see

 * Waring's Problem