Mathematics Colloquium

Gauss’s Class Number Problem

Speaker: Ken Ono, University of Virginia

Location: Warren Weaver Hall 1302

Date: Monday, March 9, 2020, 3:45 p.m.


 In 1798 Gauss wrote Disquisitiones Arithmeticae, the first rigorous text in number theory. This book laid the groundwork for modern algebraic number theory and arithmetic geometry. Perhaps the most important contribution in the work is Gauss's theory of integral quadratic forms, which appears prominently in modern number theory (sums of squares, Galois theory, rational points on elliptic curves,L-functions, the Riemann Hypothesis, to name a few). Despite the long history of the field, Gauss’s first problem about quadratic forms has not been optimally resolved. Gauss's class number problem asks for the complete list of quadratic form discriminants with class number h. The difficulty is in effective computation, which arises from the fact that the Riemann Hypothesis remains open. The subtlety of this problem is clearly illustrated in the case h=1, which was unresolved until the 1970s. In the 1980s, Goldfeld, and Gross and Zagier famously obtained the first effective class number bounds by making use of deep results on the Birch and Swinnerton-Dyer Conjecture. This lecture will tell the story of Gauss’s class number problem, and will offer some new results by the speaker and Michael Griffin that gives new effective results by different (and also more elementary) means.