Consecutive ones property and PQ-trees for multisets: Hardness of counting their orderings

  • Authors:
  • Giovanni Battaglia;Roberto Grossi;Noemi Scutellí

  • Affiliations:
  • Dipartimento di Informatica, Universití di Pisa, Italy;Dipartimento di Informatica, Universití di Pisa, Italy;List SpA, Pisa, Italy

  • Venue:
  • Information and Computation
  • Year:
  • 2012

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Abstract

A binary matrix satisfies the consecutive ones property (c1p) if its columns can be permuted such that the 1s in each row of the resulting matrix are consecutive. Equivalently, a family of setsF={Q"1,...,Q"m}, where Q"i@?R for some universe R, satisfies the c1p if the symbols in R can be permuted such that the elements of each set Q"i@?F occur consecutively, as a contiguous segment of the permutation of R@?s symbols. Motivated by combinatorial problems on sequences with repeated symbols, we consider the c1p version on multisets and prove that counting the orderings (permutations) thus generated is #P-complete. We prove completeness results also for counting the permutations generated by PQ-trees (which are related to the c1p), thus showing that a polynomial-time algorithm is unlikely to exist when dealing with multisets and sequences with repeated symbols. To prove our results, we use a combinatorial approach based on parsimonious reductions from the Hamiltonian path problem, which enables us to prove also the hardness of approximation for these counting problems.