Hilbert-Waring Theorem/Particular Cases/2

Particular Case of the Hilbert-Waring Theorem: $k = 2$
The Hilbert-Waring Theorem states that:

The case where $k = 2$ is proved by Lagrange's Four Square Theorem‎:
 * $\map g 2 = 4$

That is, every positive integer can be expressed as the sum of at most $4$ squares.