In 1638, Fermat proposed that every Positive Integer is a sum of *at most* three
Triangular Numbers, four Square Numbers, five Pentagonal
Numbers, and -Polygonal Numbers. Fermat claimed to have a proof of
this result, although Fermat's proof has never been found. Gauß proved the triangular case, and noted the event
in his diary on July 10, 1796, with the notation

This case is equivalent to the statement that every number of the form is a sum of three Odd Squares (Duke 1997). More specifically, a number is a sum of three Squares Iff it is not of the form for , as first proved by Legendre in 1798.

Euler was unable to prove the square case of Fermat's theorem, but he left partial results which were subsequently used by Lagrange. The square case was finally proved by Jacobi and independently by Lagrange in 1772. It is therefore sometimes known as Lagrange's Four-Square Theorem. In 1813, Cauchy proved the proposition in its entirety.

**References**

Cassels, J. W. S. *Rational Quadratic Forms.* New York: Academic Press, 1978.

Conway, J. H.; Guy, R. K.; Schneeberger, W. A.; and Sloane, N. J. A. ``The Primary Pretenders.''
*Acta Arith.* **78**, 307-313, 1997.

Duke, W. ``Some Old Problems and New Results about Quadratic Forms.'' *Not. Amer. Math. Soc.* **44**, 190-196, 1997.

Shanks, D. *Solved and Unsolved Problems in Number Theory, 4th ed.* New York: Chelsea, pp. 143-144, 1993.

Smith, D. E. *A Source Book in Mathematics.* New York: Dover, p. 91, 1984.

© 1996-9

1999-05-26