Elements of Finite Order for Finite Monadic Church-Rosser Thue Systems
Friedrich Otto · Transactions of the American Mathematical Society · 1985
A Thue system $T$ over $\Sigma$ is said to allow nontrivial elements of finite order, if there exist a word $u \in {\Sigma ^ \ast }$ and integers $n \ge 0$ and $k \ge 1$ such that $u leftrightarrow _T^ \ast \lambda$ and ${u^{n + k}} \leftrightarrow _T^ \ast {u^n}$. Here the following decision problem is shown to be decidable: