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 classi¯cation 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.