形式言語理論：非可換と可換のあいだ

抄録

Formal language theory is the field of study of non-commutative objects so-called “languages” (sets of words). While many problems on languages are difficult to solve due to the non-commutativity, the situation could be drastically changed if we consider a commutative invariant of a language. This paper explains some commutative invariants of languages with concrete examples.