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This is a question I have thought about and asked a number of people, but have never got an answer beyond "It should be true that..."

Given a finitely generated group $G$ (eg. a link group $G_L:=\pi_1(S^3-L)$ for a link $L$) and a finite group $H$ we want to count homomorphisms from $G$ to $H$. For link groups as above, this is an invariant of $L$.

My question: (when) for which $H$) is there a polynomial-time algorithm (in the number of generators and relations for $G$) for computing $N(G,H):=|Hom(G,H)|$ (particularly for $G_L$)?

Some things I know: 1) If $L$ is a knot and $H$ is nilpotent then $N(G_L,H)$ is constant (M. Eisermann) 2) D. Matei; A. I. Suciu, have an algorithm for solvable $H$, but the complexity is not clear. 3) The abelianization of $G_L$ is just $Z^c$, $c$ the number of components, so for $H$ abelian it is easy.

A wild conjecture is that it should always be "FPRASable" i.e. there exists a fully polynomial randomized approximation scheme for the computation.

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complexity of counting homomorphisms

This is a question I have thought about and asked a number of people, but have never got an answer beyond "It should be true that..."

Given a finitely generated group $G$ (eg. a link group $G_L:=\pi_1(S^3-L)$ for a link $L$) and a finite group $H$ we want to count homomorphisms from $G$ to $H$. For link groups as above, this is an invariant of $L$.

My question: (when) is there a polynomial-time algorithm for computing $N(G,H):=|Hom(G,H)|$ (particularly for $G_L$)?

Some things I know: 1) If $L$ is a knot and $H$ is nilpotent then $N(G_L,H)$ is constant (M. Eisermann) 2) D. Matei; A. I. Suciu, have an algorithm for solvable $H$, but the complexity is not clear. 3) The abelianization of $G_L$ is just $Z^c$, $c$ the number of components, so for $H$ abelian it is easy.

A wild conjecture is that it should always be "FPRASable" i.e. there exists a fully polynomial randomized approximation scheme for the computation.