Counting permutations by their alternating runs

Counting Permutations by Their Runs Up and DownE

Counting Permutations by Alternating Descents

We find the exponential generating function for permutations with all valleys even and all peaks odd, and use it to determine the asymptotics for its coefficients, answering a question posed by Liviu Nicolaescu. The generating function can be expressed as the reciprocal of a sum involving Euler numbers: ( 1− E1x + E3 x3 3! − E4 x4 4! + E6 x6 6! − E7 x7 7! + · · · )−1 , (∗) where ∑∞ n=0Enx n/n! ...

Enumerating Permutations by their Run Structure

Motivated by a problem in quantum field theory, we study the up and down structure of circular and linear permutations. In particular, we count the length of the (alternating) runs of permutations by representing them as monomials and find that they can always be decomposed into so-called ‘atomic’ permutations introduced in this work. This decomposition allows us to enumerate the (circular) per...

Permutations with extremal number of fixed points

Abstract. We extend Stanley’s work on alternating permutations with extremal number of fixed points in two directions: first, alternating permutations are replaced by permutations with a prescribed descent set; second, instead of simply counting permutations we study their generating polynomials by number of excedances. Several techniques are used: Désarménien’s desarrangement combinatorics, Ge...

Alternating, Pattern-Avoiding Permutations

We study the problem of counting alternating permutations avoiding collections of permutation patterns including 132. We construct a bijection between the set Sn(132) of 132-avoiding permutations and the set A2n+1(132) of alternating, 132avoiding permutations. For every set p1, . . . , pk of patterns and certain related patterns q1, . . . , qk, our bijection restricts to a bijection between Sn(...