**
Czechoslovak Mathematical Journal, Vol. 54, No. 2, pp. 465-485, 2004
**

#
On a connection of number theory

with graph theory

##
Lawrence Somer, Michal Krizek

* L. Somer*, Department of Mathematics, Catholic University of America, Washington, D.C. 20064, U.S.A., e-mail: ` somer@cua.edu`; * M. Krizek*, Mathematical Institute, Academy of Sciences of the Czech Republic, Zitna 25, CZ-115 67 Prague 1, Czech Republic, e-mail: ` krizek@math.cas.cz`

**Abstract:** We assign to each positive integer $n$ a digraph whose set of vertices is $H=\{0,1,\dots,n-1\}$ and for which there is a directed edge from $a\in H$ to $b\in H$ if $a^2\equiv b\pmod n$. We establish necessary and sufficient conditions for the existence of isolated fixed points. We also examine when the digraph is semiregular. Moreover, we present simple conditions for the number of components and length of cycles. Two new necessary and sufficient conditions for the compositeness of Fermat numbers are also introduced.

**Keywords:** Fermat numbers, Chinese remainder theorem, primality, group theory, digraphs

**Classification (MSC 2000):** 11A07, 11A15, 11A51, 05C20, 20K01

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