Proof of a conjecture of Guy on class numbers

Abstract

It is well known that for any prime $p\equiv 3$ (mod $4$), the class numbers of the quadratic fields $\mathbb{Q}(\sqrt{p})$ and $\mathbb{Q}(\sqrt{-p})$, $h(p)$ and $h(-p)$ respectively, are odd. It is natural to ask whether there is a formula for $h(p)/h(-p)$ modulo powers of $2$. We show the formula $h(p)\equiv h(-p)m(p)$ (mod $16$), where $m(p)$ is an integer defined using the "negative" continued fraction expansion of $\sqrt{p}$. Our result solves a conjecture of Richard Guy.

arXiv comment9 pages; additional background given in introduction concerning $h(p)$ and $h(-p)$ modulo small powers of 2