Hamiltonicity of regular graphs and blocks of consecutive ones in symmetric matrices

We show that the Hamiltonicity of a regular graph G can be fully characterized by the numbers of blocks of consecutive ones in the binary matrix A+ I, where A is the adjacency matrix of G, I is the unit matrix, and the blocks can be either linear or circular. Concretely, a k-regular graph G with girth g(G) 5 has a Hamiltonian circuit if and only if the matrix A + I can be permuted on rows such that each column has at most (or exactly) k− 1 circular blocks of consecutive ones; and if the graph G is k-regular except for two (k− 1)-degree vertices a and b, then there is a Hamiltonian path from a to b if and only if the matrixA+ I can be permuted on rows to have at most (or exactly) k − 1 linear blocks per column. Then we turn to the problem of determining whether a given matrix can have at most k blocks of consecutive ones per column by some row permutation. For this problem, Booth and Lueker gave a linear algorithm for k = 1 [Proceedings of the Seventh Annual ACM Symposium on Theory of Computing, 1975, pp. 255–265]; Flammini et al. showed its NP-completeness for general k [Algorithmica 16 (1996) 549–568]; and Goldberg et al. proved the same for every fixed k 2 [J. Comput. Biol. 2 (1) (1995) 139–152]. In this paper, we strengthen their result by proving that the problem remains NP-complete for every constant k 2 even if the matrix is restricted to (1) symmetric, or (2) having at most three blocks per row. © 2007 Elsevier B.V. All rights reserved.