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In mathematics and computer science, a rational series is a generalisation of the concept of formal power series over a ring to the case when the basic algebraic structure is no longer a ring but a semiring, and the indeterminates adjoined are not assumed to commute. They can be regarded as algebraic expressions of a formal language over a finite alphabet. ==Definition== Let ''R'' be a semiring and ''A'' a finite alphabet. A ''noncommutative polynomial'' over ''A'' is a finite formal sum of words over ''A''. They form a semiring . A ''formal series'' is a ''R''-valued function ''c'', on the free monoid ''A'' *, which may be written as : The set of formal series is denoted and becomes a semiring under the operations : : A non-commutative polynomial thus corresponds to a function ''c'' on ''A'' * of finite support. In the case when ''R'' is a ring, then this is the ''Magnus ring'' over ''R''. If ''L'' is a language over ''A'', regarded as a subset of ''A'' * we can form the ''characteristic series'' of ''L'' as the formal series : corresponding to the characteristic function of ''L''. In one can define an operation of iteration expressed as : and formalised as : The ''rational operations'' are the addition and multiplication of formal series, together with iteration. A rational series is a formal series obtained by rational operations from . 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Rational series」の詳細全文を読む スポンサード リンク
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