# Blatter-Specker Theorem for ternary relations

 Importance: Medium ✭✭
 Author(s): Makowsky, Janos A.
 Subject: Logic » Finite Model Theory
 Keywords: Blatter-Specker Theorem FMT00-Luminy
 Posted by: dberwanger on: May 18th, 2012

Let $C$ be a class of finite relational structures. We denote by $f_C(n)$ the number of structures in $C$ over the labeled set $\{0, \dots, n-1 \}$. For any class $C$ definable in monadic second-order logic with unary and binary relation symbols, Specker and Blatter showed that, for every $m \in \mathbb{N}$, the function $f_C(n)$ is ultimately periodic modulo $m$.

\begin{question} Does the Blatter-Specker Theorem hold for ternary relations. \end{question}

Our exposition follows closely [BS84].

\section{Counting labeled structures modulo $m$}

Let $C$ be a class of finite structures for one binary relation symbol $R$. We define for $A = \{ 1, \ldots, n \}$ $$F_C(n) = \mid \{ R^A \subseteq A^2 : \langle A, R^A \rangle \in C \} \mid$$ \paragraph{Examples:} \begin{enumerate} \item If $C=U$ consists of all $R$-structures, $f_U(n)= 2^{n^2}$. \item If $C=B$ consists of bijections, $f_B(n)= n!$ \item If $C= G$ is the class of all (undirected, simple) graphs, $f_G(n)= 2^{\binom{n}{2}}$. \item If $C=E$ is the class of all equivalence relations, then $f_E(n)= B_n$, the {\em Bell Numbers}. \item If $C=E_2$ is the class of all equivalence relations with two classes only, of the same size, $f_{E_2}(2n)= \frac{1}{2} \cdot {\binom{2n}{n}}$. Clearly, $f_{E_2}(2n+1)= 0$. \item If $C=T$ is the class of all trees, $f_T(n)= n^{n-2}$, {\em Caley}. \end{enumerate} We observe the following:

$$f_C(n)= 2^{n^2} = (-1)^{n^2} \pmod{3}$$

$$f_C(n)= n! = 0 \pmod{m} \mbox{ for } n \geq m$$

And for each $m$ the functions, $f_G(n)= 2^{\binom{n}{2}}$, $f_E(n)= B_n$, $f_T(n)= n^{n-2}$ are ultimately periodic $\pmod{m}$.

However, $f_{E_2}(2n)= \frac{1}{2} \cdot {\binom{2n}{n}} = 1 \pmod{2}$ iff $n = 2^{2k}$, hence is not periodic $\pmod{2}$.

The first four examples (all relations, all bijections, all graphs, all equivalence relations) are definable in First Order Logic $\text{FO}$. The trees are definable in Monadic Second Order Logic $\text{MSO}$..

$E_2$ is definable in Second Order Logic $\text{SO}$, but it is not $\text{MSO}$-definable. If we expand $E_2$ to have the bijection between the classes we get structures with two binary relations. The class is now $\text{FO}$-definable. Let us denote the corresponding counting function $F_{E_2}(2n)$. We have $$f_{E_2}(2n) \cdot n! = F_{E_2}(n) = 0 \pmod{m}$$ for $n$ large enough.

\section{Periodicity and linear recurrence relations} The periodicity of $f_C(n)$ $\pmod{m}$ is usually established by exhibiting a \emph{linear recurrence relation}:

There exists $1 \leq k \in \mathbb{N}$ and integers $a_1, \ldots, a_k$ such that for all $n$ $$f_C(n) = \sum_{j=1}^{k} a_j \cdot f_C(n-j) \pmod{m}$$

\paragraph{Examples.} \begin{enumerate} \item In the case of $f_C(n) = 2^{n^2}$ we have $$f_C(n) = f_C(n-2) + 2 \cdot f_C(n-1) \pmod{3}$$ \item In the case of $f_C(n) = n!$ we have for all $m$ $$f_C(n) = 0 \cdot f_C(n-1) \pmod{m}$$ In this case we say that $f_C$ \emph{trivializes}. \end{enumerate}

\section{The Blatter-Specker Theorem}

\begin{theorem}[BS84] Let $\tau$ be a binary vocabulary, i.e. all relation symbols are at most binary. If $C$ is a class of finite $\tau$-structures which is $\text{MSO}$-definable, then for all $m \in \mathbb{N}$ $f_C(n)$ is ultimately periodic $\pmod{m}$.

Moreover, there exists $1 \leq k \in \mathbb{N}$ and integers $a_1, \ldots, a_k$ such that for all $n$ $$f_C(n) = \sum_{j=1}^{k} a_j \cdot f_C(n-j) \pmod{m}$$ i.e we have a linear recurrence relation. \end{theorem}

In [F03] Fischer showed that the Specker-Blatter Theorem does not hold for quaternary relations.

The case of ternary relations remains open.

## Bibliography

% Example: %*[B] Claude Berge, Farbung von Graphen, deren samtliche bzw. deren ungerade Kreise starr sind, Wiss. Z. Martin-Luther-Univ. Halle-Wittenberg Math.-Natur. Reihe 10 (1961), 114. % %[CRS] Maria Chudnovsky, Neil Robertson, Paul Seymour, Robin Thomas: \arxiv[The strong perfect graph theorem]{math.CO/0212070}, % Ann. of Math. (2) 164 (2006), no. 1, 51--229. \MRhref{MR2233847} % % (Put an empty line between individual entries)

[BS84]* C. Blatter and E. Specker, Recurrence relations for the number of labeled structures on a finite set, Logic and Machines: Decision Problems and Complexity, E. Börger, G. Hasenjaeger and D. Rödding, eds, LNCS 171 (1984) pp. 43-61.

[F03] E. Fischer, The Specker-Blatter theorem does not hold for quaternary relations, Journal of Combinatorial Theory Series A 103(2003), 121-136.

[FM06] E. Fischer and J. A. Makowsky, The Specker-Blatter Theorem revisited: Generating functions for definable classes of stuctures. In Computing and Combinatorics (COCOON 2003) Proc., LNCS vol. 2697 (2003), 90-101.

[S88] E. Specker, Application of Logic and Combinatorics to Enumeration Problems, Trends in Theoretical Computer Science, E. Börger ed., Computer Science Press, 1988, pp. 141-169. Reprinted in: Ernst Specker, Selecta, Birkhäuser 1990, pp. 324-350.

* indicates original appearance(s) of problem.