In Wagoner, 1987 the simplicial
complex PA of Markov partitions was introduced as a tool for studying the group of
automorphisms of a subshift of finite type (XA,σA) built from a zero-one transition
matrix A. Triangles in PA led to the matrix Triangle Identities in Wagoner, Pac.
Journal, 1990 which have been used in Wagoner, 1990, 1990, 1990, 1992, Kim, Roush
& Wagoner, 1992, and the Williams Conjecture counterexample paper Kim & Roush,
to appear.
A key fact about PA is that it is contractible. See Wagoner, 1987. The purpose of
this note is to correct the proof on pp. 99-100 in Wagoner, 1987 that PA is simply
connected and in the process to improve the bound in Proposition 2.13 of Wagoner,
1987.
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