Hadwiger conjecture (graph theory)
In graph theory, the Hadwiger conjecture (or Hadwiger's conjecture) states that, if all proper colorings of an undirected graph G use k or more colors, then one can find k disjoint connected subgraphs of G such that each subgraph is connected by an edge to each other subgraph. Contracting the edges within each of these subgraphs so that each subgraph collapses to a single vertex produces a complete graph Kk on k vertices as a minor of G.
This conjecture, a far-reaching generalization of the four-color problem, was made by Hugo Hadwiger in 1943 and is still unsolved. Bollobás, Catlin & Erdős (1980) call it “one of the deepest unsolved problems in graph theory.”
An equivalent form of the Hadwiger conjecture (the contrapositive of the form stated above) is that, if there is no sequence of edge contractions (each merging the two endpoints of some edge into a single supervertex) that brings a graph G to the complete graph Kk, then G must have a vertex coloring with k − 1 colors.
Note that, in a minimal k-coloring of any graph G, contracting each color class of the coloring to a single vertex will produce a complete graph Kk. However, this contraction process does not produce a minor of G because there is (by definition) no edge between any two vertices in the same color class, thus the contraction is not an edge contraction (which is required for minors). Hadwiger's conjecture states that there exists a different way of properly edge contracting sets of vertices to single vertices, producing a complete graph Kk, in such a way that all the contracted sets are connected.
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