Lexicographic product of graphs
In graph theory, the lexicographic product or (graph) composition G ∙ H of graphs G and H is a graph such that
- the vertex set of G ∙ H is the cartesian product V(G) × V(H); and
- any two vertices (u,v) and (x,y) are adjacent in G ∙ H if and only if either u is adjacent with x in G or u = x and v is adjacent with y in H.
The lexicographic product was first studied by Felix Hausdorff (1914). As Feigenbaum & Schäffer (1986) showed, the problem of recognizing whether a graph is a lexicographic product is equivalent in complexity to the graph isomorphism problem.
The lexicographic product is in general noncommutative: G ∙ H ≠ H ∙ G. However it satisfies a distributive law with respect to disjoint union: (A + B) ∙ C = A ∙ C + B ∙ C. In addition it satisfies an identity with respect to complementation: C(G ∙ H) = C(G) ∙ C(H).
- α(G ∙ H) = α(G)α(H).
The clique number of a lexicographic product is as well multiplicative:
- ω(G ∙ H) = ω(G)ω(H).
- χ(G ∙ H) = χb(G), where b = χ(H).
- Feigenbaum, J.; Schäffer, A. A. (1986), "Recognizing composite graphs is equivalent to testing graph isomorphism", SIAM Journal on Computing, 15 (2): 619–627, doi:10.1137/0215045, MR 0837609.
- Geller, D.; Stahl, S. (1975), "The chromatic number and other functions of the lexicographic product", Journal of Combinatorial Theory, Series B, 19: 87–95, doi:10.1016/0095-8956(75)90076-3, MR 0392645.
- Hausdorff, F. (1914), Grundzüge der Mengenlehre, Leipzig
- Imrich, Wilfried; Klavžar, Sandi (2000), Product Graphs: Structure and Recognition, Wiley, ISBN 0-471-37039-8
- Ravindra, G.; Parthasarathy, K. R. (1977), "Perfect product graphs", Discrete Mathematics, 20 (2): 177–186, doi:10.1016/0012-365X(77)90056-5, MR 0491304.