2008/11/12 by Niklas Eriksen, Eriksen, Niklas, Ragnar Freij-Hollanti +5
Computer Science · Mathematics · #Advanced Combinatorial Mathematics #Computational Geometry and Mesh Generation #Optimization and Search Problems #math.CO #msc:05A05 #msc:05A15
paper · pdf · doi:10.48550/arxiv.0811.1925
arxiv created 2008/11/12 · arxiv updated 2009/12/01
We enumerate derangements with descents in prescribed positions. A generating function was given by Guo-Niu Han and Guoce Xin in 2007. We give a combinatorial proof of this result, and derive several explicit formulas. To this end, we consider fixed point λ-coloured permutations, which are easily enumerated. Several formulae regarding these numbers are given, as well as a generalisation of Euler's difference tables. We also prove that except in a trivial special case, if a permutation π is chosen uniformly among all permutations on n elements, the events that π has descents in a set S of positions, and that π is a derangement, are positively correlated.