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Hardness of Network Satisfaction for Relation Algebras with Normal Representations

We study the computational complexity of the general network satisfaction problem for a finite relation algebra $A$ with a normal representation $B$. If $B$ contains a non-trivial equivalence relation with a finite number of equivalence classes, then the network satisfaction problem for $A$ is NP-hard. As a second result, we prove hardness if $B$ has domain size at least three and contains no non-trivial equivalence relations but a symmetric atom $a$ with a forbidden triple $(a,a,a)$, that is, $a \not\leq a \circ a$. We illustrate how to apply our conditions on two small relation algebras.