Minimal permutations with d descents

  • Authors:
  • Toufik Mansour;Sherry H. F. Yan

  • Affiliations:
  • Department of Mathematics, University of Haifa, 31905 Haifa, Israel;Department of Mathematics, Zhejiang Normal University, Jinhua 321004, PR China

  • Venue:
  • European Journal of Combinatorics
  • Year:
  • 2010

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Abstract

Recently, Bouvel and Pergola initiated the study of a special class of permutations, minimal permutations with a given number of descents, which arise from the whole genome duplication-random loss model of genome rearrangement. In this paper, we show that the number of minimal permutations of length 2d-1 with d descents is given by 2^d^-^3(d-1)c"d, where c"d is the d-th Catalan number. For fixed n, we also derive a recurrence relation on the multivariate generating function for the number of minimal permutations of length n counted by the number of descents, and the values of the first and second elements of the permutation. For fixed d, on the basis of this recurrence relation, we obtain a recurrence relation on the multivariate generating function for the number of minimal permutations of length n with n-d descents, counted by the length, and the values of the first and second elements of the permutation. As a consequence, the explicit generating functions for the numbers of minimal permutations of length n with n-d descents are obtained for d@?5. Furthermore, we show that for fixed d=1, there exists a constant a"d such that the number of minimal permutations of length n with n-d descents is asymptotically equivalent to a"dd^n, as n-~.