The classical “Four Squares Theorem” of Lagrange (1770) asserts that any positive integer can be
expressed as the sum of four squares—that is, the quadratic form x
2 + y
2 + z
2 + t
2
represents all (positive)
integers. When does a general (positive definite) quadratic form represent all (positive) integers? This
question was first posed, and addressed in a systematic way, by Ramanujan in his classic 1916 paper [10].
In this paper, Ramanujan discovered and wrote down 54 more quaternary (four-variable) quadratic forms
representing all integers! His list was as follows: