The enumeration of finite rings

Abstract

Let $p$ be a fixed prime. We show that the number of isomorphism classes of finite rings of order $p^n$ is $p^{\alpha }$, where $\alpha =\frac{4}{27}{n}^3+O(n^{5/2})$. This result was stated (with a weaker error term) by Kruse and Price in 1969; a problem with their proof was pointed out by Knopfmacher in 1973. We also show that the number of isomorphism classes of finite commutative rings of order $p^n$ is $p^{\beta }$, where $\beta =\frac{2}{27}{n}^3+O(n^{5/2})$. This result was stated (again with a weaker error term) by Poonen in 2008, with a proof that relies on the problematic step in Kruse and Price's argument.

31 pages. Change of title, revised appendix, and various other small changes since previous version