Classification of subsets with minimal width and dual width in Grassmann, bilinear forms and dual polar graphs

Brouwer, Godsil, Koolen and Martin [Width and dual width of subsets in polynomial association schemes, J. Combin. Theory Ser. A 102 (2003) 255–271] introduced the width w and the dual width w * of a subset in a distance-regular graph and in a cometric association scheme, respectively, and then derived lower bounds on these new parameters. For instance, subsets with the property w + w * = d in a cometric distance-regular graph with diameter d attain these bounds. In this paper, we classify subsets with this property in Grassmann graphs, bilinear forms graphs and dual polar graphs. We use this information to establish the Erdős–Ko–Rado theorem in full generality for the first two families of graphs.