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The Grötzsch graph is a triangle-free graph that cannot be colored with fewer than four colors. Much research about triangle-free graphs has focused on graph coloring. Every bipartite graph (that is, every 2-colorable graph) is triangle-free, and Grötzsch's theorem states that every triangle-free planar graph may be 3-colored. [8]
The concept was introduced by William K. Hale. [2] If T = {0} it reduces to common vertex coloring. The T-chromatic number, (), is the minimum number of colors that can be used in a T-coloring of G. The complementary coloring of T-coloring c, denoted ¯ is defined for each vertex v of G by
Outerplanar graphs have a forbidden graph characterization analogous to Kuratowski's theorem and Wagner's theorem for planar graphs: a graph is outerplanar if and only if it does not contain a subdivision of the complete graph K 4 or the complete bipartite graph K 2,3. [3] Alternatively, a graph is outerplanar if and only if it does not contain ...
In the mathematical field of graph theory, the Grötzsch graph is a triangle-free graph with 11 vertices, 20 edges, chromatic number 4, and crossing number 5. It is named after German mathematician Herbert Grötzsch, who used it as an example in connection with his 1959 theorem that planar triangle-free graphs are 3-colorable.
The empty graph E 3 (red) admits a 1-coloring; the complete graph K 3 (blue) admits a 3-coloring; the other graphs admit a 2-coloring. Main article: Chromatic polynomial The chromatic polynomial counts the number of ways a graph can be colored using some of a given number of colors.
Locally cyclic graphs other than K 4 are exactly the underlying graphs of Whitney triangulations, embeddings of graphs on surfaces in such a way that the faces of the embedding are the cliques of the graph. [3] Locally cyclic graphs can have as many as () edges. [4] Claw-free graphs are the graphs that are locally co-triangle-free; that is, for ...
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