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Counting induced subgraphs: a topological approach to #W[1]-hardness. (English) Zbl 1452.68086

Summary: We investigate the problem \(\# \mathsf{IndSub} (\Phi)\) of counting all induced subgraphs of size \(k\) in a graph \(G\) that satisfy a given property \(\Phi\). This continues the work of M. Jerrum and K. Meeks who proved the problem to be #W[1]-hard for some families of properties which include (dis)connectedness [J. Comput. Syst. Sci. 81, No. 4, 702–716 (2015; Zbl 1320.68101)] and even- or oddness of the number of edges [Combinatorica 37, No. 5, 965–990 (2017; Zbl 1413.68063)]. Using the recent framework of graph motif parameters due to [R. Curticapean et al., in: Proceedings of the 49th annual ACM SIGACT symposium on theory of computing, STOC’17. New York, NY: Association for Computing Machinery (ACM). 210–223 (2017; Zbl 1369.05191)], we discover that for monotone properties \(\Phi\), the problem \(\# \mathsf{IndSub} (\Phi)\) is hard for #W[1] if the reduced Euler characteristic of the associated simplicial (graph) complex of \(\Phi\) is non-zero. This observation links \(\# \mathsf{IndSub} (\Phi)\) to Karp’s famous Evasiveness Conjecture, as every graph complex with non-vanishing reduced Euler characteristic is known to be evasive. Applying tools from the “topological approach to evasiveness” which was introduced in the seminal paper of J. Kahn et al. [Combinatorica 4, 297–306 (1984; Zbl 0577.05061)], we prove that \(\# \mathsf{IndSub} (\Phi)\) is #W[1]-hard for every monotone property \(\Phi\) that does not hold on the Hamilton cycle as well as for some monotone properties that hold on the Hamilton cycle such as being triangle-free or not \(k\)-edge-connected for \(k > 2\). Moreover, we show that for those properties \(\# \mathsf{IndSub} (\Phi)\) can not be solved in time \(f(k) \cdot n^{o(k)}\) for any computable function \(f\) unless the Exponential Time Hypothesis (ETH) fails. In the final part of the paper, we investigate non-monotone properties and prove that \(\# \mathsf{IndSub} (\Phi)\) is #W[1]-hard if \(\Phi\) is any non-trivial modularity constraint on the number of edges with respect to some prime \(q\) or if \(\Phi\) enforces the presence of a fixed isolated subgraph.

MSC:

68Q17 Computational difficulty of problems (lower bounds, completeness, difficulty of approximation, etc.)
05C30 Enumeration in graph theory
05C60 Isomorphism problems in graph theory (reconstruction conjecture, etc.) and homomorphisms (subgraph embedding, etc.)
05E45 Combinatorial aspects of simplicial complexes
68Q27 Parameterized complexity, tractability and kernelization

Software:

OEIS

References:

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