Inversion sequences of length $n$, $\mathbf{I}_n$, are integer sequences $(e_1, \ldots, e_n)$ with $0 \leq e_i < n$ for each $i$. The study of patterns in inversion sequences was initiated recently by Mansour-Shattuck and Corteel-Martinez-Savage-Weselcouch through a systematic study of inversion sequences avoiding words of length 3. We continue this investigation by generalizing the notion of a pattern to a fixed triple of binary relations $(\rho_1,\rho_2,\rho_3)$ and consider the set $\mathbf{I}_n(\rho_1,\rho_2,\rho_3)$ consisting of those $e \in \mathbf{I}_n$ with no $i < j < k$ such that $e_i \rho_1 e_j$, $e_j \rho_2 e_k$, and $e_i \rho_3 e_k$. We show that "avoiding a triple of relations" can characterize inversion sequences with a variety of monotonicity or unimodality conditions, or with multiplicity constraints on the elements. We uncover several interesting enumeration results and relate pattern avoiding inversion sequences to familiar combinatorial families. We highlight open questions about the relationship between pattern avoiding inversion sequences and families such as plane permutations and Baxter permutations. For several combinatorial sequences, pattern avoiding inversion sequences provide a simpler interpretation than otherwise known.

Patterns in Inversion Sequences II: Inversion Sequences Avoiding Triples of Relations

Enumerating five families of pattern-avoiding inversion sequences; and introducing the powered Catalan numbers

Patterns of relation triples in inversion and ascent sequences

In this paper, we enumerate two families of pattern-avoiding permutations: those avoiding the vincular pattern $2-41-3$, which we call semi-Baxter permutations, and those avoiding the vincular patterns $2-41-3$, $3-14-2$ and $3-41-2$, which we call strong-Baxter permutations. We call semi-Baxter numbers and strong-Baxter numbers the associated enumeration sequences. We prove that the semi-Baxter numbers enumerate in addition plane permutations (avoiding $2-14-3$). The problem of counting these permutations was open and has given rise to several conjectures, which we also prove in this paper. 
For each family (that of semi-Baxter -- or equivalently, plane -- and that of strong-Baxter permutations), we describe a generating tree, which translates into a functional equation for the generating function. For semi-Baxter permutations, it is solved using (a variant of) the kernel method: this gives an expression for the generating function while also proving its D-finiteness. From the obtained generating function, we derive closed formulas for the semi-Baxter numbers, a recurrence that they satisfy, as well as their asymptotic behavior. For strong-Baxter permutations, we show that their generating function is (a slight modification of) that of a family of walks in the quarter plane, which is known to be non D-finite.

pdf/semi-baxter-and-strong-baxter-two-relatives-of-the-baxter-3oyge0a955.pdf

Semi-Baxter and strong-Baxter: two relatives of the Baxter sequence

The first problem addressed by this article is the enumeration of some families of pattern-avoiding inversion sequences. We solve some enumerative conjectures left open by the foundational work on the topics by Corteel et al., some of these being also solved independently by Lin, and Kim and Lin. The strength of our approach is its robustness: we enumerate four families $F_1 \subset F_2 \subset F_3 \subset F_4$ of pattern-avoiding inversion sequences ordered by inclusion using the same approach. More precisely, we provide a generating tree (with associated succession rule) for each family $F_i$ which generalizes the one for the family $F_{i-1}$. 
The second topic of the paper is the enumeration of a fifth family $F_5$ of pattern-avoiding inversion sequences (containing $F_4$). This enumeration is also solved \emph{via} a succession rule, which however does not generalize the one for $F_4$. The associated enumeration sequence, which we call the \emph{powered Catalan numbers}, is quite intriguing, and further investigated. We provide two different succession rules for it, denoted $\Omega_{pCat}$ and $\Omega_{steady}$, and show that they define two types of families enumerated by powered Catalan numbers. Among such families, we introduce the \emph{steady paths}, which are naturally associated with $\Omega_{steady}$. They allow us to bridge the gap between the two types of families enumerated by powered Catalan numbers: indeed, we provide a size-preserving bijection between steady paths and valley-marked Dyck paths (which are naturally associated with $\Omega_{pCat}$). 
Along the way, we provide several nice connections to families of permutations defined by the avoidance of vincular patterns, and some enumerative conjectures.

pdf/enumerating-five-families-of-pattern-avoiding-inversion-378i0wjs1i.pdf

In this paper, we enumerate two families of pattern-avoiding permutations: those avoiding the vincular pattern $2\underbracket{41}3$, which we call semi-Baxter permutations, and those avoiding the ...

Semi-Baxter and Strong-Baxter: Two Relatives of the Baxter Sequence

Fighting fish were very recently introduced by the authors as combinatorial structures made of square tiles that form two dimensional branching surfaces. A main feature of these fighting fish is that the area of uniform random fish of size $n$ scales like $n^{5/4}$ as opposed to the typical $n^{3/2}$ area behavior of the staircase or direct convex polyominoes that they generalize. In this extended abstract we concentrate on enumerative properties of fighting fish: in particular we provide a new decomposition and we show that the number of fighting fish with $i$ left lower free edges and $j$ right lower free edges is equal to 
\begin{equation*} 
\frac{(2i+j-2)!(2j+i-2)!}{i!j!(2i-1)!(2j-1)!}. 
\end{equation*} These numbers are known to count rooted planar non-separable maps with $i+1$ vertices and $j+1$ faces, or two-stack-sortable permutations with respect to ascending and descending runs, or left ternary trees with respect to vertices with even and odd abscissa. However we have been unable until now to provide any explicit bijection between our fish and such structures. Instead we provide new refined generating series for left ternary trees to prove further equidistribution results.

Fighting Fish: enumerative properties

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Semi-Baxter and Strong-Baxter: Two Relatives of the Baxter Sequence

Enumerating five families of pattern-avoiding inversion sequences; and introducing the powered Catalan numbers

Semi-Baxter and strong-Baxter: two relatives of the Baxter sequence

Enumerating five families of pattern-avoiding inversion sequences; and introducing the powered Catalan numbers

Fighting Fish: enumerative properties