We consider functions of natural numbers which allow a combinatorial interpretation as density functions (speed) of classes of relational structures, s uch as Fibonacci numbers, Bell numbers, Catalan numbers and the like. Many of these functions satisfy a linear recurrence relation over $mathbb Z$ or ${mathbb Z}_m$ and allow an interpretation as counting the number of relations satisfying a property expressible in Monadic Second Order Logic (MSOL). C. Blatter and E. Specker (1981) showed that if such a function $f$ counts the number of binary relations satisfying a property expressible in MSOL then $f$ satisfies for every $m in mathbb{N}$ a linear recurrence relation over $mathbb{Z}_m$. In this paper we give a complete characterization in terms of definability in MSOL of the combinatorial functions which satisfy a linear recurrence relation over $mathbb{Z}$, and discuss various extensions and limitations of the Specker-Blatter theorem.
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