20 research outputs found
A Survey of Alternating Permutations
This survey of alternating permutations and Euler numbers includes
refinements of Euler numbers, other occurrences of Euler numbers, longest
alternating subsequences, umbral enumeration of classes of alternating
permutations, and the cd-index of the symmetric group.Comment: 32 pages, 7 figure
Pattern avoiding alternating involutions
We enumerate and characterize some classes of alternating and reverse
alternating involutions avoiding a single pattern of length three or four. If
on one hand the case of patterns of length three is trivial, on the other hand,
the length four case is more challenging and involves sequences of
combinatorial interest, such as Motzkin and Fibonacci numbers
Enumeration of derangements with descents in prescribed positions
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 -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 is chosen uniformly among all
permutations on elements, the events that has descents in a set
of positions, and that is a derangement, are positively correlated
Enumeration of derangements with descents in prescribed positions
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 -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 is chosen uniformly among all permutations on elements, the events that has descents in a set of positions, and that is a derangement, are positively correlated