Thickness (graph theory)

In graph theory, the thickness of a graph is the minimum number of planar graphs into which the edges of can be partitioned. That is, if there exists a collection of planar graphs, all having the same set of vertices, such that the union of these planar graphs is , then the thickness of is at most . In other words, the thickness of a graph is the minimal number of planar subgraphs whose union equals to graph .