Volume 30 • Number 1 • 2007   • Coalescence of Difans and Diwheels Diego Bravo and Juan Rada Abstract. A directed graph G is nonderogatory if its adjacency matrix A is nonderogatory, i.e., the characteristic polynomial of A is equal to the minimal polynomial of A. We analyze the problem whether the coalescence of difans and diwheels is nonderogatory. Also, a formula for the characteristic polynomial of the coalescence of two directed graphs is presented. 2000 Mathematics Subject Classification:05C50. Full text: PDF