A Wilf equivalence related to two stack sortable permutations

Comtet introduced the notion of indecomposable permutations in 1972. A permutation is indecomposable if and only if it has no proper prefix which is itself a permutation. Indecomposable permutations were studied in the literature in various contexts. In particular, this notion has been proven to be useful in obtaining non-trivial enumeration and equidistribution results on permutations. In this paper, we give a complete classification of indecomposable permutations avoiding...

We consider the enumeration of pattern-avoiding involutions, focusing in particular on sets defined by avoiding a single pattern of length 4. As we demonstrate, the numerical data for these problems demonstrates some surprising behavior. This strange behavior even provides some very unexpected data related to the number of 1324-avoiding permutations.

We show that the counting sequence for permutations avoiding both of the (classical) patterns 1243 and 2134 has the algebraic generating function supplied by Vaclav Kotesovec for sequence A164651 in The On-Line Encyclopedia of Integer Sequences.

We consider the problem of enumerating permutations with exactly r occurrences of the pattern 1324 and derive functional equations for this general case as well as for the pattern avoidance (r=0) case. The functional equations lead to a new algorithm for enumerating length n permutations that avoid 1324. This approach is used to enumerate the 1324-avoiders up to n=31. We also extend those functional equations to account for the number of inversions and derive analogous algori...

Let S_n(321) (respectively, S_n(132)) denote the set of all permutations of {1,2,...,n} that avoid the pattern 321 (respectively, the pattern 132). Elizalde and Pak gave a bijection Theta from S_n(321) to S_n(132) that preserves the numbers of fixed points and excedances for each element of S_n(321), and commutes with the operation of taking inverses. Bloom and Saracino proved that another bijection Gamma from S_n(321) to S_n(132), introduced by Robertson, has the same proper...

In this paper, we find explicit formulas or generating functions for the cardinalities of the sets $S_n(T,\tau)$ of all permutations in $S_n$ that avoid a pattern $\tau\in S_k$ and a set $T$, $|T|\geq 2$, of patterns from $S_3$. The main body of the paper is divided into three sections corresponding to the cases $|T|=2,3$ and $|T|\geq 4$. As an example, in the fifth section, we obtain the complete classification of all cardinalities of the sets $S_n(T,\tau)$ for $k=4$.

We confirm a conjecture of Lara Pudwell and show that permutations of [n] that avoid the barred pattern bar{3}bar{1}542 are counted by OEIS sequence A047970. In fact, we show bijectively that the number of bar{3}bar{1}542 avoiders of length n with j+k left-to-right maxima, of which j initiate a descent in the permutation and k do not, is {n}-choose-{k} j! StirlingPartition{n-j-k}{j}, where StirlingPartition{n}{j} is the Stirling partition number.

In this paper, we study the Wilf-type equivalence relations among multiset permutations. We identify all multiset equivalences among pairs of patterns consisting of a pattern of length three and another pattern of length at most four. To establish our results, we make use of a variety of techniques, including Ferrers-equivalence arguments, sorting by minimal/maximal letters, analysis of active sites and direct bijections. In several cases, our arguments may be extended to pro...

In this paper, we prove several enumeration results of pattern-avoiding Fishburn permutations that Egge (arXiv:2208.01484) proposed recently. Our results include enumerating Fishburn permutations avoiding 321 and one of the following three types of patterns: a classical pattern of size 4, two classical patterns of size 4, or a classical pattern of size 5.

In recent work, Zeilberger and the author used a functional equations approach for enumerating permutations with r occurrences of the pattern 12...k. In particular, the approach yielded a polynomial-time enumeration algorithm for any fixed nonnegative r. We extend that approach to patterns of the form 12...(k-2)(k)(k-1) by deriving analogous functional equations and using them to develop similar algorithms that enumerate permutations with r occurrences of the pattern. We also...