Flow equivalence of sofic shifts

The Inverse Hull of 0-Left Cancellative Semigroups

“…notice that if a is any member of Σ, then either 10 n 4a, or 20 n 4a is a forbidden word, according to whether a is of the form [0, 2, 3, 4] or [0, 1,3,4]. As a consequence we deduce that 0 n 4a does not belong to F {1,2} , for any a in Σ, whence 0 n 4 lies in the boundary ∂F {1,2} .…”