The number of linear extensions of the tree.
We use Knuth's hook length formula for trees [pg.70, 1]. For an ordered tree $T$ on $n$ vertices, the number of linear extensions is
$$ \frac{n!}{\prod_{v\in T}|T_v|}, $$
where $T_v$ is the number of vertices of the subtree rooted at $v$.

Sends a permutation to its associated increasing tree.
This tree is recursively obtained by sending the unique permutation of length $0$ to the empty tree, and sending a permutation $\sigma$ of length $n \geq 1$ to a root node with two subtrees $L$ and $R$ by splitting $\sigma$ at the index $\sigma^{-1}(1)$, normalizing both sides again to permutations and sending the permutations on the left and on the right of $\sigma^{-1}(1)$ to the trees $L$ and $R$, respectively.

Return an ordered tree of size $n+1$ by the following recursive rule:

• if $x$ is the left child of $y$, $x$ becomes the left brother of $y$,
• if $x$ is the right child of $y$, $x$ becomes the last child of $y$.

Sends a Dyck path to a 132-avoiding permutation.
This bijection is defined in [1, Section 2].