Binary codes that do not preserve primitivity

TL;DR

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Contribution

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Abstract

A code $X$ is not primitivity preserving if there is a primitive list ${\mathbf w} \in {\tt lists} X$ whose concatenation is imprimitive. We formalize a full characterization of such codes in the binary case in the proof assistant Isabelle/HOL. Part of the formalization, interesting on its own, is a description of $\{x,y\}$-interpretations of the square $xx$ if $|y| \leq |x|$. We also provide a formalized parametric solution of the related equation $x^jy^k = z^\ell$.