Dissecting a Square to Four Squares
This Demonstration shows how to dissect a square into four squares on the basis of the identity .
This Demonstration illustrates examples of Lagrange's four-square theorem, which states that every positive integer can be written as the sum of at most four squares. The first published proof was given by Lagrange (1736–1813) in 1770.
 G. N. Frederickson, Dissections: Plane & Fancy, New York: Cambridge University Press, 2002 pp. 88 and 282.
 Wikipedia. "Lagrance's Four-Square Theorem." (Feb 22, 2013) en.wikipedia.org/wiki/Lagrange%27s_four-square_theorem.