An extension of the bivariate chromatic polynomial

Abstract

K. Dohmen, A. Pönitz and P. Tittmann [K. Dohmen, A. Pönitz, P. Tittmann, A new two-variable generalization of the chromatic polynomial, Discrete Mathematics and Theoretical Computer Science 6 (2003), 69–90], introduced a bivariate generalization of the chromatic polynomial P ( G , x , y ) which subsumes also the independent set polynomial of I. Gutman and F. Harary [I. Gutman, F. Harary, Generalizations of the matching polynomial, Utilitas Mathematicae 24 (1983), 97–106] and the vertex-cover polynomial of F.M. Dong, M.D. Hendy, K.T. Teo and C.H.C. Little [F.M. Dong, M.D. Hendy, K.L. Teo, and C.H.C. Little, The vertex-cover polynomial of a graph, Discrete Mathematics 250 (2002), 71–78]. We first show that P ( G , x , y ) has a recursive definition with respect to three kinds of edge eliminations: edge deletion, edge contraction, and edge extraction, i.e. deletion of an edge together with its endpoints. Like in the case of deletion and contraction only [J.G. Oxley and D.J.A. Welsh, The Tutte polynomial and percolation, in: J.A. Bundy, U.S.R. Murty (Eds.), Graph Theory and Related Topics, Academic Press, London, 1979, pp. 329–339] it turns out that there is a most general, or as they call it, a universal polynomial satisfying such recurrence relations with respect to the three kinds of edge eliminations, which we call ξ ( G , x , y , z ) . We show that the new polynomial simultaneously generalizes, P ( G , x , y ) , as well as the Tutte polynomial and the matching polynomial, We also give an explicit definition of ξ ( G , x , y , z ) using a subset expansion formula. We also show that ξ ( G , x , y , z ) can be viewed as a partition function, using counting of weighted graph homomorphisms. Furthermore, we expand this result to edge-labeled graphs as was done for the Tutte polynomial by T. Zaslavsky [T. Zaslavsky, Strong Tutte functions of matroids and graphs, Trans. Amer. Math. Soc. 334 (1992), 317–347] and by B. Bollobás and O. Riordan [B. Bollobás, O. Riordan, A Tutte polynomial for coloured graphs, Combinatorics, Probability and Computing 8 (1999), 45–94]. The edge-labeled polynomial ξ lab ( G , x , y , z , t ̄ ) also generalizes the chain polynomial of R.C. Read and E.G. Whitehead Jr. [R.C. Read, E.G. Whitehead Jr., Chromatic polynomials of homeomorphism classes of graphs, Discrete Mathematics 204 (1999), 337–356]. Finally, we discuss the complexity of computing ξ ( G , x , y , z ) .