ing in the seventeenth century with Fermat’s assertions of 1640 about the
numbers represented by x
2 + y
2
. In the next century, Euler gave proofs of
these and some similar assertions about other simple binary quadratics, and
although these proofs had some gaps, they contributed greatly to setting
the theory on a firm foundation.
Lagrange started the theory of universal quadratic forms in 1770 by
proving his celebrated Four Squares Theorem, which in current language is
expressed by saying that the form x
2+y
2+z
2+t
2
is universal. The eighteenth
century was closed by a considerably deeper statement – Legendre’s Three
Squares Theorem of 1798; this found exactly which numbers needed all four
squares. In his Theorie des Nombres of 1830, Legendre also created a very
general theory of binary quadratics.
The new century was opened by Gauss’s Disquisitiones Arithmeticae of
1801, which brought that theory to essentially its modern state. Indeed,
when Neil Sloane and I wanted to summarize the classification theory of
binary forms for one of our books [3], we found that the only Number Theory
textbook in the Cambridge Mathematical Library that handled every case
was still the Disquisitiones! Gauss’s initial exploration of ternary quadratics
was continued by his great disciple Eisenstein, while Dirichlet started the
analytic theory by his class number formula of 1839.
As the nineteenth century wore on, other investigators, notably H. J. S.
Smith and Hermann Minkowski, explored the application of Gauss’s concept
of the genus to higher-dimensional forms, and introduced some invariants
for the genus from which in this century Hasse was able to obtain a complete