On the road coloring problem

Author:
Joel Friedman

Journal:
Proc. Amer. Math. Soc. **110** (1990), 1133-1135

MSC:
Primary 05C15

MathSciNet review:
953004

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Abstract: Let be a strongly connected, aperiodic, directed graph having outdegree 2 at each vertex. A *red-blue coloring* of is a coloring of the edges with the colors red and blue such that each vertex has one red edge and one blue edge leaving it. Given such a coloring, we define by iff there is a red edge from to . Similarly we define . is said to be collapsible if some composition of 's and 's maps to a single vertex. The road coloring problem is to determine whether has a collapsible coloring. It has been conjectured that all such have a collapsible coloring. Since has outdegree 2 everywhere and is strongly connected, the adjacency matrix, , of has a positive left eigenvector with eigenvalue 2 , i.e. . Furthermore, we can assume that 's components are integers with no common factor. We call the *weight* of . Let , defined to be the weight of the graph. We will prove that if has a simple cycle of length relatively prime to , then is collapsibly colorable.

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DOI:
http://dx.doi.org/10.1090/S0002-9939-1990-0953004-8

Article copyright:
© Copyright 1990
American Mathematical Society