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\begin{document}
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\theoremstyle{plain}
\newtheorem{theorem}{Theorem}
\newtheorem{corollary}[theorem]{Corollary}
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{proposition}[theorem]{Proposition}
\theoremstyle{definition}
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\begin{center}
\vskip 1cm{\LARGE\bf
Two Permutation Classes Enumerated by the \\
\vskip .12in
Central Binomial Coefficients
}
\vskip 1cm
\large
Marilena Barnabei and Flavio Bonetti \\
Dipartimento di Matematica \\
Universit\`a di Bologna\\
Piazza di Porta San Donato 5 \\
40126 Bologna \\
Italy\\
\href{mailto:marilena.barnabei@unibo.it}{\tt marilena.barnabei@unibo.it} \\
\href{mailto:flavio.bonetti@unibo.it}{\tt flavio.bonetti@unibo.it}\\
\ \\
Matteo Silimbani\\
LaBRI --- Universit\'e Bordeaux 1\\
351, cours de la Lib\'eration \\
33405 Talence \\
France\\
\href{mailto:matteo.silimbani@labri.fr}{\tt matteo.silimbani@labri.fr} \\
\end{center}
\vskip .2 in
\begin{abstract} We define a map between the set of permutations that avoid
either the four patterns $3214,3241,4213,4231$ or
$3124,3142,4123,4132$, and the set of Dyck prefixes. This map,
when restricted to either of the two classes, turns out to be a
bijection that allows us to determine some notable features of
these permutations, such as the distribution of the statistics
``number of ascents'', ``number of left-to-right maxima'', ``first
element'', and ``position of the maximum element''.\end{abstract}
\section{Introduction}
The well-known pattern containment order over the set of
permutations is defined as follows: a permutation $\sigma$
\emph{contains} a permutation $\tau$ if there exists a subsequence
of $\sigma$ that has the same relative order as $\tau$, and in
this case $\tau$ is said to be a \emph{pattern} occurring in
$\sigma$. Otherwise, $\sigma$ is said to \emph{avoid the pattern}
$\tau$.
A \emph{class} of permutations is a downset in the
permutation pattern containment order. Every class can be defined
by its basis $B$, namely, the set of minimal permutations that are
not contained in it; the class is denoted by $\Av(B)$. We let
$S_n(B)$ denote the set $\Av(B)\cap S_n$, where $S_n$ is the set of all
permutations of length $n$.
The classes of permutations that avoid one or more
patterns of length $3$ have been exhaustively studied since the
seminal paper \cite{simisch}. In many cases, the properties of
these permutations have been determined by establishing suitable
bijections with lattice paths (see the paper by Claesson and Kitaev \cite{ck} for a survey).
The case of patterns of length $4$ still seems to be incomplete,
even with regard to the mere enumeration in the case of multiple
avoidance (for a comprehensive overview on the subject see the book by Kitaev \cite[Chapter\ 6.1]{kitbook}. See also the paper by Mansour and Vainshtein \cite{mava} for a case of multiple avoidance concerning the pattern $132$ and other patterns of length $4$ or more).
Guibert \cite{guib} dealt with some enumerative problems concerning multiple avoidance in his Ph.D. thesis.
In particular, \cite[Theorem\ 4.6]{guib} exhibits $12$ different classes of permutations avoiding four patterns of length $4$,
each one enumerated by
the sequence of central binomial coefficients.
On the other hand, it is well known that the central binomial coefficient ${2n\choose n}$ (see \seqnum{A000984} in \cite{oeis}) enumerates the set of Dyck prefixes of length $2n$, namely, lattice paths in the integer
lattice $\mathbb{N}\times\mathbb{N}$ starting from the origin,
consisting of up steps $U=(1,1)$ and down steps $D=(1,-1)$, and
never passing below the $x$-axis.
In this paper we consider two of Guibert's classes,
namely, \begin{eqnarray*}\Av(T_1)=\Av(3214,3241,4213,4231) & \text{ and }&
\Av(T_2)=\Av(3124,3142,4123,4132),\end{eqnarray*} and introduce a map $\Phi$
between the union of these classes and the set of Dyck prefixes.
The restrictions of this map to the sets $\Av(T_1)$ and $\Av(T_2)$
turn out to be bijections. The key tool in determining this map is
Theorem \ref{pincopallo}, which describes the structure of
permutations of both classes. If we consider the decomposition
$\sigma=\alpha\,n\,\beta$, where $n$ is the maximum symbol in
$\sigma$ and $\alpha$ and $\beta$ are possibly empty words, then
if the permutation $\sigma$ avoids the four patterns in $T_1$, the
prefix $\alpha$ avoids $321$ and the suffix $\beta$ avoids both
$231$ and $213$, while if $\sigma$ avoids $T_2$, the prefix
$\alpha$ avoids $312$ and the suffix $\beta$ avoids both $123$ and
$132$, where $\alpha$ and $\beta$ satisfy some additional
constraints described in the next section.
In both cases, the lattice path $\Phi(\sigma)$ is obtained by first associating with $\alpha$ a Dyck prefix by a
procedure similar to the one used by Krattenthaler \cite{kratt}, and then appending to this prefix a
sequence of up steps and down steps that depends on the suffix $\beta$.
The map $\Phi$ allows us to relate some properties of a permutation $\sigma$ with some particular features of the corresponding Dyck prefix $P$. For example, the Dyck prefix $P$ does not touch the $x$-axis (except for the origin)
whenever $\sigma$ is connected (see Section \ref{jjjj}), while $P$ is a Dyck path whenever $\sigma$ ends with the maximum symbol. Moreover, if $\sigma\in \Av(T_1)$, the $y$-coordinate of the last point of $P$ gives information about the maximum length of a decreasing subsequence in $\sigma$.
The map $\Phi$ allows us also to prove that each one of
the three statistics ``number of left-to-right maxima'',
``position of the maximum element'', and ``first element'' are
equidistributed over the two classes, and we determine the
generating function of these statistics. Finally, in the last
section we determine the distribution of the statistic ``number of
ascents'', which is not equidistributed over the two classes.
\section{The classes $\Av(3214,3241,4213,4231)$ and \\ $\Av(3124,3142,4123,4132)$}
In this paper we are interested in the two classes
$\Av(T_1)$ and $\Av(T_2)$, where $T_1=\{3214,3241,4213,4231\}$ and
$T_2=\{3124,3142,4123,4132\}$. First of all, we characterize the
permutations in these classes by means of their
left-to-right-maxima decomposition.
Recall that a permutation $\sigma$ has a
\emph{left-to-right maximum} at position $i$ if
$\sigma(i)\geq\sigma(j)$ for every $j\leq i$, and that every
permutation $\sigma$ can be decomposed as
$$\sigma=M_1\,w_1\,M_2\,w_2\,\cdots\,M_k\,w_k,$$
where $M_1,\ldots,M_k$ are the left-to-right maxima of $\sigma$
and $w_1,\ldots,w_k$ are (possibly empty) words. A
characterization of permutations in $\Av(T_1)$ and $\Av(T_2)$ is easily deduced as follows:
\newtheorem{inizio}{Theorem}
\begin{inizio}\label{pincopallo}
Let $\sigma\in S_n$. Consider the decomposition
$$\sigma=M_1\,w_1\,M_2\,w_2\,\cdots\,M_k\,w_k,$$ where $M_k=n$.
Let $l_i$ denote the length of the word $w_i$. Then
\begin{itemize}
\item[a.] $\sigma$
belongs to $\Av(T_1)$ if and only if
\begin{itemize} \item the
reduced form of $w_k$ is a permutation in $\Av(231,213)$, and
\item if $k>1$, the juxtaposition of the words $w_1,\ldots,w_{k-1}$
consists of the smallest $l_1+\cdots+l_{k-1}$ symbols in the set
$[n-1]\setminus\{M_1,\ldots,M_{k-1}\}$, listed in increasing
order. In particular, the permutation\\
$\alpha=M_1\,w_1\,M_2\,w_2\,\cdots\,M_{k-1}\,w_{k-1}$, after
reduced form, avoids $321$.
\end{itemize}
\item[b.] $\sigma$
belongs to $\Av(T_2)$ if and only if
\begin{itemize} \item the
reduced form of $w_k$ is a permutation in $\Av(123,132)$, and
\item if $k>1$, every word $w_i$, $i\leq k-1$, consists of the $l_i+1$ greatest unused symbols among those that are less than $M_i$, listed in decreasing order.
In particular the reduced form of
$\alpha=M_1\,w_1\,M_2\,w_2\,\cdots\,M_{k-1}\,w_{k-1}$ avoids
$312$.
\end{itemize}
\end{itemize}\end{inizio}
This result implies that a permutation $\sigma\in S_n(T_1)$ can be decomposed into
$$\sigma=\alpha\, n \, \beta,$$
where $\alpha$ avoids $321$ and $\beta$ avoids both $231$ and $213$.
Similarly, a permutation $\sigma'\in S_n(T_2)$ can be decomposed into
$$\sigma'=\alpha'\, n \, \beta',$$
where $\alpha'$ avoids $312$ and $\beta'$ avoids both $123$ and
$132$. We note that, in both cases, this property is not a
characterization, since the permutations $\alpha$ and $\alpha'$
can not be chosen arbitrarily, according to Theorem
\ref{pincopallo}: for example, the permutation $3\,2\,4\,1$, that
belongs to $T_1$, has precisely the described structure.
It is easy to see that a permutation
$\tau=x_1\,x_2\,\ldots\,x_j$ belongs to $\Av(231,213)$ whenever,
for every $i\leq j$, the integer $x_i$ is either the minimum or
the maximum of the set $\{x_i,x_{i+1},\ldots,x_j\}$. Analogously,
$\tau$ belongs to $\Av(123,132)$ whenever, for every $i\leq j$, the
integer $x_i$ is either the greatest or the second greatest
element of the set $\{x_i,x_{i+1},\ldots,x_j\}$ (see, e.g.,
\cite{simisch}).
For example, if we consider the permutation in $S_{10}(T_1)$
$$\sigma=4\,1\,2\,6\,7\,3\,10\,5\,9\,8,$$ we have
$$\alpha=4\,1\,2\,6\,7\,3,$$
with
$$M_1=4\quad M_2=6\quad M_3=7\quad M_4=10,$$
and
$$\beta=5\,9\,8.$$
Analogously, if we consider the permutation in $S_{10}(T_2)$
$$\tau=4\,3\,2\,6\,7\,5\,10\,8\,9\,1,$$ we have
$$\alpha=4\,3\,2\,6\,7\,5,$$
with
$$M_1=4\quad M_2=6\quad M_3=7\quad M_4=10,$$
and
$$\beta=8\,9\,1.$$
The preceding considerations provide a characterization of the permutations in the two classes that end with the maximum symbol. Let $B_n$ denote the set of permutations in $S_n$ ending by $n$, by $B^{(1)}_n=B_n\cap \Av(T_1)$, and by $B^{(2)}_n=B_n\cap \Av(T_2)$. Theorem \ref{pincopallo} yields immediately the
following result:
\newtheorem{biie}[inizio]{Corollary}
\begin{biie}\label{forseavn}
The function $\psi_n:B_n\to S_{n-1}$ that maps a permutation
$\sigma$ into the permutation in $S_{n-1}$ obtained by deleting
the last symbol in $\sigma$ yields a bijection between \begin{itemize}
\item $B^{(1)}_n$ and
$S_{n-1}(321)$;
\item $B^{(2)}_n$ and
$S_{n-1}(312)$.
\end{itemize}
\end{biie}
\section{Bijections with Dyck prefixes}
\label{jjjj}
A \emph{Dyck prefix} is a lattice path in the integer
lattice $\mathbb{N}\times\mathbb{N}$ starting from the origin,
consisting of up steps $U=(1,1)$ and down steps $D=(1,-1)$, and
never passing below the $x$-axis. Obviously, a Dyck prefix can be
also seen as a word $W$ in the alphabet $\{U,D\}$ such that
every initial subword of $W$ contains at least as many symbols $U$
as symbols $D$.
It is well known (see, e.g., \cite{vanl}) that the
number of Dyck prefixes of length $n$ is $\left({n\atop
\left\lfloor\frac{n}{2}\right\rfloor}\right)$.
A Dyck prefix ending at ground level is a \emph{Dyck
path}.
We now define a map $\Phi:\Av(T_1)\cup \Av(T_2)\to \mathscr{P}$, where
$\mathscr{P}$ is the set of Dyck prefixes of even length.
First of all, associate the permutation $\sigma=1$ with the empty
path. Then, for every $n\geq 1$, associate a permutation
$\sigma\in S_{n+1}(T_1)\cup S_{n+1}(T_2)$ with a Dyck prefix of length $2n$, as
follows. Set $\sigma=M_1\,w_1\,M_2\,w_2\,\cdots\,M_k\,w_k$ as
above and let $l_i$ be the length of the word $w_i$.
\begin{itemize}
\item If $w_k$ is empty, then $$\Phi(\sigma)=U^{M_1}D^{l_1+1}U^{M_2-M_1}D^{l_2+1}\cdots
U^{M_{k-1}-M_{k-2}}D^{l_{k-1}+1};$$
\item If $w_k=x_1\cdots x_{l_k}$ is not empty, then
$$\Phi(\sigma)=U^{M_1}D^{l_1+1}U^{M_2-M_1}D^{l_2+1}\cdots
U^{M_{k}-M_{k-1}}Q,$$ where $Q$ is the sequence $Q_1\cdots
Q_{l_k-1}$ of $l_k-1$ steps such that, for every
$j$, $Q_j$ is an up step if
$x_j=\max{\{x_j,x_{j+1},\ldots,x_{l_k}\}}$, a down step otherwise.
\end{itemize}
We point out that, in both cases, the last element of $\sigma$
is not processed. It is easy to check that the word $\Phi(\sigma)$
is a Dyck prefix of length $2n$.
For example, consider the permutation in $S_{12}(T_1)$
$$\sigma=6\ 1\ 2\ 9\ 3\ 4\ 5\ 11\ 12\ 7\ 10\ 8.$$
We have $M_1=6$, $w_1=1\ 2$, $M_2=9$, $w_2=3\ 4\ 5$, $M_3=11$,
$w_3$ is empty, $M_4=12$, and $w_4=7\ 10\ 8$. The Dyck prefix
$\Phi(\sigma)$ is shown in Figure \ref{info}.
\begin{figure}[ht]
\begin{center}
\includegraphics[bb=77 592 488 719,width=.6\textwidth]{esebij} \caption{The
Dyck prefix $\Phi(6\ 1\ 2\ 9\ 3\ 4\ 5\ 11\ 12\ 7\ 10\
8)$.}\label{info}
\end{center}
\end{figure}
Note that, given a permutation $\sigma\in S_{n+1}(T_1)\cup S_{n+1}(T_2)$, $\sigma=M_1\,w_1\,\cdots\,M_k\,w_k$, the
position of the symbol $n+1$ (that is related to the existence and position of the $(n+1)$-th
up step in the associated Dyck prefix) plays an important role
in the definition of the map $\Phi$. For this reason, the $(n+1)$-th
up step in $\Phi(\sigma)$, if any, will be called the \emph{cut
step} of the path. Needless to say, $\Phi(\sigma)$ contains a cut
step if and only if it is not a Dyck path. In fact, if $w_k$ is empty, the path
$\Phi(\sigma)$ contains $M_{k-1}=n$ up steps, hence it is a Dyck
path. On the other hand, if $w_k$ is not empty, $\Phi(\sigma)$
contains at least $M_{k}=n+1$ up steps, therefore it does not end
at the ground level. The preceding considerations can be
summarized as follows:
\newtheorem{bleah}[inizio]{Proposition}
\begin{bleah}\label{hoepli}
Consider a permutation $\sigma\in S_{n+1}(T_1)\cup S_{n+1}(T_2)$.
Then, $\Phi(\sigma)$ is a Dyck path if and only if
$\sigma(n+1)=n+1$.
\end{bleah}
Now let $\Phi_1$ and $\Phi_2$ denote the two restrictions of
$\Phi$ to the sets $\Av(T_1)$ and $\Av(T_2)$. Our next goal is to
prove that the two restrictions $\Phi_1$ and $\Phi_2$ are indeed
bijections, by defining their inverses as follows. Both
$\Phi_1^{-1}$ and $\Phi_2^{-1}$ associate the empty path with the
permutation $1$. Consider now a Dyck prefix
$P=U^{h_1}D^{s_1}U^{h_2}D^{s_2}\cdots U^{h_r}D^{s_r}$ of length
$2n$, $n\geq 1$. The permutation $\sigma=\Phi_1^{-1}(P)$ is
defined as follows:
\begin{itemize}
\item If $P$ is a Dyck path, namely, $h_1+\cdots +h_r=n=s_1+\cdots +s_r$,
set \begin{itemize}\item[]$\sigma(1)=h_1,$
\item[]$\sigma(s_1+1)=h_1+h_2,$
\item[]$\vdots$\item[] $\sigma(s_1+\cdots+s_{r-1}+1)=n,$
\item[]$\sigma(n+1)=n+1.$\end{itemize}
Then, place the remaining symbols in increasing order in
the unassigned positions.
\item If $P$ is not a Dyck path, let $t$ denote the index of the ascending run $U^{h_t}$ containing the cut step. \begin{itemize} \item[-] if $t=1$, $P$ decomposes into
$$P=U^{n+1}Q,$$
where $Q$ is a lattice path. In this case, set
$$\sigma(1)=n+1;$$
\item[-] if $t>1$, $P$ decomposes into
$$P=U^{h_1}D^{s_1}\cdots
U^{h_t}D^{s_t}U^{n+1-h_1-\cdots-h_t}Q.$$ Set\begin{itemize}
\item[] $\sigma(1)=h_1,$
\item[] $\sigma(s_1+1)=h_1+h_2,$
\item[] $\vdots$
\item[] $\sigma(s_1+\cdots+s_{t-1}+1)=n+1.$\end{itemize}
\end{itemize} In both cases, set $i=s_1+\cdots+s_{t-1}+1$ (or $i=1$ if $t=1$). Fill the unassigned positions less
than $i$ with the smallest remaining symbols placed in increasing
order. Then, for every $j=1,\ldots,n-i$, set either
\begin{itemize}
\item[] $\sigma(i+j)=\min [n+1]\setminus\{\sigma(1),\sigma(2),\ldots,\sigma(i+j-1)\} $ if
the $j$-th step of the path $Q$ is a down step, or
\item[] $\sigma(i+j)=\max [n+1]\setminus\{\sigma(1),\sigma(2),\ldots,\sigma(i+j-1)\}$ if
the $j$-th step of the path $Q$ is an up step.
\end{itemize}
Finally, $\sigma(n+1)$ equals the last unassigned symbol.
\end{itemize}
The permutation $\tau=\Phi_2^{-1}(P)$ can be defined
similarly:
\begin{itemize}
\item If $P$ is a Dyck path
set \begin{itemize}\item[]$\tau(1)=h_1;$
\item[]$\tau(s_1+1)=h_1+h_2,$
\item[]$\vdots$\item[] $\tau(s_1+\cdots+s_{r-1}+1)=n,$
\item[]$\tau(n+1)=n+1.$\end{itemize}
Then, scan the unassigned positions from left to right and fill them with the greatest unused symbol among those that are less then the closest preceding left-to-right maximum.
\item If $P$ is not a Dyck path, let $t$ denote the index of the ascending run $U^{h_t}$ containing the cut step. \begin{itemize} \item[-] if $t=1$, $P$ decomposes into
$$P=U^{n+1}Q,$$
where $Q$ is a lattice path. In this case, set
$$\tau(1)=n+1;$$
\item[-] if $t>1$, $P$ decomposes into
$$P=U^{h_1}D^{s_1}\cdots
U^{h_t}D^{s_t}U^{n+1-h_1-\cdots-h_t}Q.$$ Set\begin{itemize}
\item[] $\tau(1)=h_1,$
\item[] $\tau(s_1+1)=h_1+h_2,$
\item[] $\vdots$
\item[] $\tau(s_1+\cdots+s_{t-1}+1)=n+1.$\end{itemize}
\end{itemize} In both cases, set $i=s_1+\cdots+s_{t-1}+1$ (or $i=1$ if $t=1$). Then, scan the unassigned positions less then $i$ from left to right and fill them with the greatest unused symbol among those that are less than the closest preceding left-to-right maximum. Then, for every $j=1,\ldots,n-i$, set either
\begin{itemize}
\item[] $\tau(i+j)=\max \left([n+1]\setminus\{\tau(1),\tau(2),\ldots,\tau(i+j-1)\}\right)$ if
the $j$-th step of the path $Q$ is an up step, or
\item[] $\tau(i+j)=$ the second greatest element in the set \\ $[n+1]\setminus\{\tau(1),\tau(2),\ldots,\tau(i+j-1)\} $ if
the $j$-th step of the path $Q$ is a down step.
\end{itemize}
Finally, $\tau(n+1)$ equals the last unassigned symbol.
\end{itemize}
Theorem \ref{pincopallo} ensures that $\sigma$ belongs to $S_{n+1}(T_1)$, while $\tau$ belongs to $S_{n+1}(T_2)$. Moreover,
it is easily seen that $\Phi_1^{-1}(\Phi_1(\sigma))=\sigma$ and $\Phi_2^{-1}(\Phi_2(\tau))=\tau$. As an
immediate consequence, we have the following result:
\newtheorem{numero}[inizio]{Theorem}
\begin{numero}
The two maps $\Phi_1$ and $\Phi_2$ are bijections. Hence, the cardinality of both $S_{n+1}(T_1)$ and $S_{n+1}(T_2)$ is the central binomial
coefficient ${2n\choose n }$.\end{numero}
We observe that the enumerative result contained in this theorem can be also deduced from the results by Guibert \cite[Theorem\ 4.6]{guib}.
In the following, we show that some properties of the permutations in $\Av(T_1)$ and $\Av(T_2)$ can be deduced
from certain features of the corresponding Dyck prefix.
First of all, a permutation $\sigma\in S_n$ is
\emph{connected} if it does not have a proper prefix of length
$l1$, this is equivalent to:
\begin{equation}h_{n,k}=h_{n,k-1}-h_{n-1,k-2},\label{equino}\end{equation}
with the convention $h_{s,0}=0$ for every integer $s$.
The special cases $k=1$ and $k=n$ can be easily handled as follows. First of all, the permutations
in $S_{n}(T_1)$ such that $\sigma(1)=1$ correspond to the Dyck prefixes of length $2n-2$ starting with $UD$, which are in one-to-one correspondence with Dyck prefixes
of length $2n-4$. Hence, $$h_{n,1}={2n-4\choose n-2}.$$
On the other hand, we observe that, given a Dyck prefix of length $2n-2$ starting with $U^n$, we can change
the $n-$th up step into a down step, obtaining a new lattice path that is still a Dyck
prefix. This implies that there are as many Dyck prefixes of
length $2n-2$ starting with $U^n$ as those starting with
$U^{n-1}D$, namely, $h_{n,n-1}=h_{n,n}$. Recall that permutations $\sigma\in
S_n(T_1)$ such that $\sigma(1)=n$ are in bijection with
permutations in $S_{n-1}(213,231)$. It is well known that the number of such permutations is $2^{n-2}$ (see \cite{simisch}). Hence,
$$h_{n,n-1}=h_{n,n}=2^{n-2}.$$
\newtheorem{incanato}[inizio]{Theorem}
\begin{incanato}
We have
$$H(x,y)=\frac{xy\left[(xy-1)^2(1-y)\sqrt{1-4x}+x(1-2xy)\right]}{(1-y+xy^2)(1-2xy)\sqrt{1-4x}}.$$
\end{incanato}
\begin{proof} Formula (\ref{equino}) gives a recurrence
for the integers $h_{n,k}$ for every $n\geq 3$ and $2\leq k\leq
n-1$. This fact suggests to consider first the generating function
$$G(x,y)=\sum_{n\geq 2}\sum_{k=1}^{n-1}h_{n,k}x^ny^k.$$
Formula (\ref{jkjk}) yields
$$G(x,y)=\sum_{n\geq 3}\sum_{k=2}^{n-1}h_{n,k-1}x^ny^k-\sum_{n\geq 3}\sum_{k=2}^{n-1}h_{n-1,k-2}x^ny^k+\sum_{n\geq 2}h_{n,1}x^ny=$$
$$=y\left(G(x,y)-x^2y-\sum_{n\geq 3}h_{n,n-1}x^ny^{n-1}\right)-xy^2\left(G(x,y)-\sum_{n\geq 2}h_{n,n-1}x^ny^{n-1}\right)+$$
$$+\sum_{n\geq 2}h_{n,1}x^ny.$$
The previous considerations allow us to deduce
$$(1-y+xy^2)G(x,y)=\frac{x^3y^3-x^2y^2}{1-2xy}+\frac{x^2y}{\sqrt{1-4x}}.$$
Now, $H(x,y)$ can be obtained from $G(x,y)$ as follows:
$$H(x,y)=G(x,y)+xy+\sum_{n\geq 2} 2^{n-2}x^ny^n=G(x,y)+\frac{xy-x^2y^2}{1-2xy}.$$
\end{proof}
\section{Other statistics over $\Av(T_1)$ and $\Av(T_2)$}
This section is devoted to the study of some permutation statistics that are not equidistributed over the two classes. In both cases, we will translate occurrences of permutation statistics into configurations of the corresponding path.
First of all, we recall that a permutation $\sigma$ has an
\emph{ascent} at position $i$ whenever $\sigma(i)>\sigma(i+1)$,
and let $\asc(\sigma)$ denote the number of ascents of $\sigma$.
\subsection{The class $\Av(T_1)$}
We consider the generating function of the distribution of ascents over $\Av(T_1)$:
$$F(x,y)=\sum_{n\geq 1}\sum_{\sigma\in S_n(T_1)} x^ny^{\asc(\sigma)}.$$
The ascents of $\sigma\in \Av(T_1)$ can be recovered from the Dyck
prefix $\Phi_1(\sigma)$ as follows:
\newtheorem{collant}[inizio]{Proposition}
\begin{collant}
The number of ascents of a permutation $\sigma\in \Av(T_1)$ is the
sum of:
\begin{itemize}
\item The number of valleys and the number of triple descents (i.e.,
occurrences of $DDD$) preceding the cut step in $\Phi_1(\sigma)$ (if $\Phi_1(\sigma)$ is
a Dyck path, its final down step counts as a valley); and
\item The number of down steps
following the cut step in $\Phi_1(\sigma)$.
\end{itemize}
\end{collant}
\begin{proof} Decompose $\sigma$ as
$$\sigma=M_1\,w_1\,M_2\,w_2\,\cdots\,M_k\,w_k,$$ where $M_1,\ldots,M_k$ are the left-to-right maxima of $\sigma$.
Theorem \ref{pincopallo} implies that an ascent can occur in
$\sigma$ only in one of the following positions:
\begin{itemize}
\item between two consecutive symbols in $w_i$, $i\leq
k-1$. These two symbols correspond to two consecutive down steps
in $\Phi_1(\sigma)$ coming before the cut step. By the definition of
$\Phi_1$ these two down steps are necessarily preceded by a further
down step;
\item before every left-to-right
maximum $M_i$, except for the first one. These positions
correspond exactly to the valleys of $\Phi_1(\sigma)$ coming before
the cut step. In the special case when $\sigma$ ends with its
maximum symbol, the final ascent of $\sigma$ corresponds to the
final down step of the Dyck path $\Phi_1(\sigma)$;
\item in $w_k$, every time that the minimum unassigned symbol is
chosen. These ascents are of course in bijection with the down
steps following the cut step.
\end{itemize}
\end{proof}
Also in this case, we study the distribution of ascents on the set $B^{(1)}_n$ of permutations in $\Av(T_1)$ such that $\Phi_1(\sigma)$ is a Dyck path, and on the set $C_n$ of connected permutations in $\Av(T_1)$ that correspond to floating Dyck prefixes. Afterwards, we study the behavior of the ascent distribution with respect to the last return decomposition of the corresponding path. Arguments similar to those used in the proof of Proposition \ref{nonmesso} lead to
\newtheorem{amal}[inizio]{Proposition}
\begin{amal}\label{deev}
Consider a permutation $\sigma\in \Av(T_1)$. Suppose that
$\Phi_1(\sigma)$ can be decomposed into
$$\Phi_1(\sigma)=P'\,P'',$$
where $P'$ is a non-empty Dyck path and $P''$ is any Dyck prefix.
Set $\tau=\Phi_1^{-1}(P')$ and $\rho=\Phi_1^{-1}(P'')$. Then
$$\asc(\sigma)=\asc(\tau)+\asc(\rho).$$
\end{amal}
Consider the generating function of the ascent distribution over the set $B^{(1)}_n$:
$$E(x,y)=\sum_{n\geq 1}\sum_{\sigma\in B^{(1)}_n} x^ny^{\asc(\sigma)}.$$
The present authors \cite{bbs} determined the
generating function $A(x,y,z)$ of the joint distribution of
valleys and triple descents over the set of Dyck paths, namely,
\begin{eqnarray}
A(x,y,z) & = & \sum_{n\geq 0}\sum_{P\in \mathcal{P}_n}x^ny^{v(P)}z^{td(P)}=\nonumber\\ & = & \frac{1}{2xy(xyz-z-xy)}\left(-1+xy+2x^2y\right.
\nonumber\\
& &-2x^2y^2+xz-2xyz-2x^2yz+2x^2y^2z\\
& &
\left.+\sqrt{1-2xy-4x^2y+x^2y^2-2xz+2x^2yz+x^2z^2}\right)\nonumber
\end{eqnarray}
where $\mathcal{P}_n$ is the set of Dyck paths of semilenght $n$,
$v(P)$ denotes the number of valleys of the path $P$, and $td(P)$
is the number of occurrences of $DDD$ in $P$.
We infer
\begin{equation}E(x,y)=xy(A(x,y,y)-1)+x.\label{wewewei}\end{equation}
The last summand in Formula (\ref{wewewei}) takes into account the permutation $1$. This implies that:
\newtheorem{teorem}[inizio]{Proposition}
\begin{teorem}
We have\begin{equation}
E(x,y)=\frac{\sqrt{1-4xy+4x^2y(y-1)}-1}{2y(x(y-1)-1)}.\label{dentro}
\end{equation}
\end{teorem}
Consider now the generating function of the ascent
distribution over the set $C_n$ of connected permutations in
$\Av(T_1)$.
$$V(x,y)=\sum_{n\geq 2}\sum_{\sigma\in
C_n}x^ny^{\asc(\sigma)}.$$
Proposition \ref{deev} yields the following functional equation
involving the generating functions $F(x,y)$, $E(x,y)$, and
$V(x,y)$:
\begin{equation} F(x,y)=E(x,y)+\frac{E(x,y)V(x,y)}{x}.\label{alfauno}\end{equation}
Finally, we express the generating function $V(x,y)$ in
terms of $F(x,y)$ and $E(x,y)$. Given any Dyck prefix $P$, we can obtain two Dyck prefixes $P_U$ and $P_D$ by prepending to $P$ an up step and appending either an up or a down step, as explained in the previous section and shown in Figure \ref{ehsi}.
In this case, we have
\begin{itemize}
\item If $P$ is floating, then both $P_U$ and $P_D$ are floating, and
$$\asc(\sigma_U)=h,\qquad \asc(\sigma_D)=h+1;$$
\item If $P$ is a Dyck path, only the path $P_U$ is floating, and
$$\asc(\sigma_U)=h.$$
\end{itemize}
Then, we have
\begin{equation}V(x,y)=(x+xy)F(x,y)-xyE(x,y).\label{betauno}\end{equation}
Now, exploiting Identities (\ref{alfauno}) and
(\ref{betauno}), we get the following expression of $F(x,y)$ in terms
of $E(x,y)$:
\newtheorem{plinpluc}[inizio]{Theorem}
\begin{plinpluc} We have
\begin{equation}F(x,y)=\frac{E(x,y)(1-yE(x,y))}{1-E(x,y)-yE(x,y)}.\label{taggaladai}\end{equation}
\end{plinpluc}
An explicit expression for $F(x,y)$ can be obtained by
combining Identities (\ref{dentro}) and (\ref{taggaladai}).
In the remaining of this subsection, we characterize the permutations in $\Av(T_1)$ according to the height of the last point of the path $\Phi_1(\sigma)$. Proposition \ref{hoepli} characterizes permutations in
$\Av(T_1)$ whose associated prefix ends at the ground level. Now we
characterize permutations $\sigma \in S_n(T_1)$ whose corresponding
path $\Phi_1(\sigma)$ ends at $(2n-2,2h)$, $h>0$.
First of all, it is well known that the number of Dyck
prefixes of length $2n-2$ ending at $(2n-2,2h)$ is
\begin{equation}{2n-3\choose n-1-h}-{2n-3\choose
n-3-h},\label{finisci}\end{equation}
(see \cite{vanl} and \seqnum{A039599} in \cite{oeis}).
\newtheorem{sequenza}[inizio]{Theorem}
\begin{sequenza}\label{longest}
Let $\sigma$ be a permutation in $\Av(T_1)$ not ending with the
maximum symbol. If the $y$-coordinate of the last point of the
Dyck prefix $\Phi_1(\sigma)$ is $2k-2$, then the longest decreasing subsequence of $\sigma$ has cardinality $k$.
\end{sequenza}
\begin{proof} Recall that every permutation $\sigma\in S_n(T_1)$ can be
decomposed as follows:
$$\sigma=\alpha\,n\,\beta,$$
where $\alpha$ avoids $321$ and $\beta$ avoids $213$ and
$231$. Moreover, $\beta=x_1\,x_2\,\cdots\,x_j$ is such that the
integer $x_i$ is either the minimum or the maximum of the set
$\{x_i,x_{i+1},\ldots,x_j\}$. Let $x_{i_1},\ldots,x_{i_q}$ denote
the subsequence of $\beta$ consisting of the integers $x_i$ ($1\leq
i\leq j-1$) such that $x_i$ is the maximum of the set
$\{x_i,x_{i+1},\ldots,x_j\}$. By definition of the bijection
$\Phi$, it is immediately seen that the $y$-coordinate of the last
point of $\Phi_1(\sigma)$ is
$$j+1+q-(j-1-q)=2q+2.$$
It is easy to check that the sequence
$$n\,x_{i_1}\,\cdots\,x_{i_q}\,x_j,$$ of length $k=q+2$, is the longest decreasing
subsequence in $\sigma$. This ends the proof.
\end{proof}
The preceding result allows us to characterize the set
of Dyck prefixes of length $2n-2$ corresponding via $\Phi_1$ to
permutations in $S_n(T_1)$ that avoid also the pattern
$k\,\,k-1\,\cdots\,2\,1$:
\newtheorem{gigigi}[inizio]{Theorem}
\begin{gigigi}
We have
$$|S_n(T_1,k\,\,k-1\,\cdots\,2\,1)|={2n-2 \choose n-1}-{2n-2\choose n-k}$$
\end{gigigi}
\begin{proof} The preceding results yield immediately:
$$|S_n(T_1,k\,\,k-1\,\cdots\,2\,1)|=\sum_{i=0}^{k-2}{2n-3 \choose n-1-i}-{2n-3\choose n-3-i}=$$$$={2n-2 \choose n-1}-{2n-2\choose n-k}.$$
\end{proof}
In particular, consider the case $k=3$. Of course, we have $S_n(T_1,321)=S_n(321)$. The set of Dyck prefixes of length $2n-2$
corresponding via $\Phi_1$ to permutations in
$S_n(321)$ can be partitioned into two subsets:
\begin{itemize}
\item[a)] the set of Dyck paths;
\item[b)] the set of Dyck prefixes ending at $(2n-2,2)$.
\end{itemize}
It is well known that the set $S_n(321)$ is enumerated by $n$-th Catalan number. Many bijections between permutations in
$S_n(321)$ and Dyck paths of semilength $n$ appear in the literature, notably the bijection defined by
Krattenthaler \cite{kratt}. If $\sigma$ is a permutation in $S_n(321)$, the relation between the Dyck prefix $\Phi_1(\sigma)$ and the Dyck path
$K(\sigma)$ associated with $\sigma$ by Krattenthaler's bijection can be described as follows:
\begin{itemize}
\item If $\Phi_1(\sigma)$ is a Dyck path, $K(\sigma)=\Phi_1(\sigma)UD$;
\item If the last point of $\Phi_1(\sigma)$ has coordinates $(2n-2,2)$, $K(\sigma)=\Phi_1(\sigma)DD$.
\end{itemize}
\section{The class $\Av(T_2)$}
In this last section, we study the generating function of the ascent distribution over $\Av(T_2)$
$$M(x,y)=\sum_{n\geq 1}\sum_{\sigma\in S_n(T_2)} x^ny^{\asc(\sigma)}.$$
\newtheorem{kio}[inizio]{Proposition}
\begin{kio}
The number of ascents in a permutation $\sigma\in \Av(T_2)$ is the
number of peaks in the Dyck prefix $\Phi_2(\sigma)$.
\end{kio}
\begin{proof} Decompose $\sigma$ as
$$\sigma=M_1\,w_1\,M_2\,w_2\,\cdots\,M_k\,w_k,$$ where $M_1,\ldots,M_k$ are the left-to-right maxima of $\sigma$.
Theorem \ref{pincopallo} implies that an ascent can occur in
$\sigma$ only in one of the following positions:
\begin{itemize}
\item Before every left-to-right
maximum $M_i$, except for the first one. These positions
correspond exactly to the peaks of $\Phi_2(\sigma)$ coming before
the cut step.
\item In $w_k$, every time that the second greatest unassigned symbol $x_j$ is
chosen either immediately after the cut step or immediately after the choice of the maximum unassigned element $x_{j-1}$. This means that there is an element $p$ such that $x_j