The determinant of the adjacency matrix of a graph.
For a labelled graph $G$ with vertices $\{1,\ldots,n\}$, the adjacency matrix is the matrix $(a_{ij})$ with $a_{ij} = 1$ if the vertices $i$ and $j$ are joined by an edge in $G$.
Since the determinant is invariant under simultaneous row and column permutations, the determinant of the adjacency is well-defined for an unlabelled graph.
According to [2], Equation 8, this determinant can be computed as follows: let $s(G)$ be the number of connected components of $G$ that are cycles and $r(G)$ the number of connected components that equal $K_2$. Then $$\det(A) = \sum_{H} (-1)^{r(H)} 2^{s(H)}$$ where the sum is over all spanning subgraphs $H$ of $G$ that have as connected components only $K_2$'s and cycles.

An alternative to the Adin-Bagno-Roichman transformation of a Dyck path.
This is a bijection preserving the number of up steps before each peak and exchanging the number of components with the position of the last double rise.

The incomparability graph of a poset.

The cell poset of the parallelogram polyomino corresponding to the Dyck path.
Let $D$ be a Dyck path of semilength $n$. The parallelogram polyomino $\gamma(D)$ is defined as follows: let $\tilde D = d_0 d_1 \dots d_{2n+1}$ be the Dyck path obtained by prepending an up step and appending a down step to $D$. Then, the upper path of $\gamma(D)$ corresponds to the sequence of steps of $\tilde D$ with even indices, and the lower path of $\gamma(D)$ corresponds to the sequence of steps of $\tilde D$ with odd indices.
This map returns the cell poset of $\gamma(D)$. In this partial order, the cells of the polyomino are the elements and a cell covers those cells with which it shares an edge and which are closer to the origin.