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 A-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.
|Julkaisu||The Electronic Journal of Combinatorics|
|Tila||Julkaistu - 4 maalisk. 2009|
|OKM-julkaisutyyppi||A1 Julkaistu artikkeli, soviteltu|