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 Counting subrings of the ring ${\Bbb Z}_m \times {\Bbb Z}_n$ J. Korean Math. Soc.Published online 2019 Apr 10 Laszlo Toth University of Pecs Abstract : Let $m,n\in {\Bbb N}$. We represent the additive subgroups of the ring ${\Bbb Z}_m \times {\Bbb Z}_n$, which are also (unital) subrings, and deduce explicit formulas for $N^{(s)}(m,n)$ and $N^{(us)}(m,n)$, denoting the number of subrings of the ring ${\Bbb Z}_m \times {\Bbb Z}_n$ and its unital subrings, respectively. We show that the functions $(m,n)\mapsto N^{(s)}(m,n)$ and $(m,n)\mapsto N^{(us)}(m,n)$ are multiplicative, viewed as functions of two variables, and their Dirichlet series can be expressed in terms of the Riemann zeta function. We also establish an asymptotic formula for the sum $\sum_{m,n\le x} N^{(s)}(m,n)$, the error term of which is closely related to the Dirichlet divisor problem. Keywords : subgroup; subring; ideal; number of subrings; multiplicative arithmetic function of two variables, asymptotic formula, Dirichlet divisor problem MSC numbers : Primary 11N45, 20K27; Secondary 11A25, 13A99 Full-Text :