arXiv - CS - Discrete Mathematics Pub Date : 2020-03-27 , DOI: arxiv-2003.12213
Benjamin Wells

Let $\begin{array}{c}\overline{A} = ABCBA\ CBC\ ABCBA,\\ \overline{B} = BCACB\ ACA\ BCACB,\\ \overline{C} = CABAC\ BAB\ CABAC. \end{array}$ The Leech sequence $L$ is the squarefree sequence obtained as the limit of the palindromes $A, \overline{A}, \overline{\overline{A}}, \ldots .$ In order to specify a certain class of pseudorecursive varieties of semigroups, it is helpful to have a squarefree term in 3 variables such that no substitution instance occurs as a subterm of $L$. We show that $\kappa_1 = aba\ cbc\ aba\ c$ is such a term. Except for one situation, the doubly-linked term $\kappa_2 = aba\ cbc\ aba$ will serve, and we focus on it.

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