  The Picard group of an undirected graph is a finitely generated abelian
group, and the Jacobian is the torsion subgroup of the Picard group. These
groups can be computed by using the Smith normal form of the Laplacian matrix
of the graph or by using chip-firing games associated with the graph. One may
consider its generalization to directed graphs based on the Laplacian matrix.
We compute Picard groups and Jacobians for several classes of directed trees,
cycles, wheel, and multipartite graphs.
