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A graph is called 1-planar if it can be drawn in the plane so that each of its edges is crossed by at most one other edge. We show that every 1-planar drawing of any 1-planar graph on $n$ vertices has at most $n-2$ crossings; moreover, this bound is tight. By this novel necessary condition for 1-planarity, we characterize the 1-planarity of Cartesian product $K_m\times P_n$. Based on this condition, we also derive an upper bound on the number of edges of bipartite 1-planar graphs, and we show that each subgraph of an optimal 1-planar graph (i.e., a 1-planar graph with $n$ vertices and $4n-8$ edges) can be decomposed into a planar graph and a forest.

A strong edge coloring of a graph is a proper edge coloring where the edges at distance at most two receive distinct colors. It is known that every planar graph with maximum degree ∆ has a strong edge coloring with at most 4 ∆ + 4 colors. We show that 3 ∆ + 6 colors suffice if the graph has girth 6, and 3 ∆ colors suffice if the girth is at least 7. Moreover, we show that cubic planar graphs with girth at least 6 can be strongly edge colored with at most 9 colors.