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Rational series : ウィキペディア英語版
Rational series
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 R\langle A \rangle.
A ''formal series'' is a ''R''-valued function ''c'', on the free monoid ''A''
*
, which may be written as
:\sum_ c(w) w \ .
The set of formal series is denoted R\langle\langle A \rangle\rangle and becomes a semiring under the operations
:c+d : w \mapsto c(w) + d(w) \
:c\cdot d : w \mapsto \sum_ c(u) \cdot d(v) \ .
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
:\sum_ w \
corresponding to the characteristic function of ''L''.
In R\langle\langle A \rangle\rangle one can define an operation of iteration expressed as
: S^
* = \sum_ S^n \
and formalised as
:c^
*(w) = \sum_ c(u_1)c(u_2) \cdots c(u_n) \ .
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 R\langle A \rangle.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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