We consider the two-variable interlace polynomial introduced by Arratia,
Bollobas and Sorkin (2004). We develop graph transformations which allow us to
derive point-to-point reductions for the interlace polynomial. Exploiting these
reductions we obtain new results concerning the computational complexity of
evaluating the interlace polynomial at a fixed point. Regarding exact
evaluation, we prove that the interlace polynomial is #P-hard to evaluate at
every point of the plane, except on one line, where it is trivially polynomial
time computable, and four lines, where the complexity is still open. This
solves a problem posed by Arratia, Bollobas and Sorkin (2004). In particular,
three specializations of the two-variable interlace polynomial, the
vertex-nullity interlace polynomial, the vertex-rank interlace polynomial and
the independent set polynomial, are almost everywhere #P-hard to evaluate, too.
For the independent set polynomial, our reductions allow us to prove that it is
even hard to approximate at any point except at 0.Comment: 18 pages, 1 figure; new graph transformation (adding cycles) solves
some unknown points, error in the statement of the inapproximability result
fixed; a previous version has appeared in the proceedings of STACS 200
