On symmetry in strings, sequences and languages
Solomon Marcus, Alexandru Mateescu, Gheorghe Pǎun, Arto K. Salomaa · International Journal of Computer Mathematics · 1994
Starting from the concept of symmetry in algebra and mathematical analysis, we consider the property of a string to remain "unchanged" (member of a given language) by systematicaly interchanging the occurrences of two given symbols. Extensions to sets of symbols, then the symmetric closure and the symmetric kernel of a language are defined and studied, as well as the symmetry of bi-infinite sequences