A d‐interval is the union of d disjoint intervals on the real line. A d‐track interval is the union of d disjoint intervals on d disjoint parallel lines called tracks, one interval on each track. As generalizations of the ubiquitous interval graphs, d‐interval graphs and d‐track interval graphs have wide applications, traditionally to scheduling and resource allocation, and more recently to bioinformatics. In this paper, we prove that recognizing d‐track interval graphs is NP‐complete for any constant d≥2. This confirms a conjecture of Gyárfás and West in 1995. Previously only the complexity of the case d=2 was known. Our proof in fact implies that several restricted variants of this graph recognition problem, i.e., recognizing balanced d‐track interval graphs, unit d‐track interval graphs, and (2,…,2) d‐track interval graphs, are all NP‐complete. This partially answers another question recently raised by Gambette and Vialette. We also prove that recognizing depth‐two 2‐track interval graphs is NP‐complete, even for the unit case. In sharp contrast, we present a simple linear‐time algorithm for recognizing depth‐two unit d‐interval graphs. These and other results of ours give partial answers to a question of West and Shmoys in 1984 and a similar question of Gyárfás and West in 1995. Finally, we give the first bounds on the track number and the unit track number of a graph in terms of the number of vertices, the number of edges, and the maximum degree, and link the two numbers to the classical concepts of arboricity.
