# Rational series

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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 $R\langle A\rangle$ .

A formal series is a R-valued function c, on the free monoid A*, which may be written as

$\sum _{w\in A^{*}}c(w)w\ .$ The set of formal series is denoted $R\langle \langle A\rangle \rangle$ and becomes a semiring under the operations

$c+d:w\mapsto c(w)+d(w)$ $c\cdot d:w\mapsto \sum _{uv=w}c(u)\cdot d(v)\ .$ 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

$\sum _{w\in L}w$ corresponding to the characteristic function of L.

In $R\langle \langle A\rangle \rangle$ one can define an operation of iteration expressed as

$S^{*}=\sum _{n\geq 0}S^{n}$ and formalised as

$c^{*}(w)=\sum _{u_{1}u_{2}\cdots u_{n}=w}c(u_{1})c(u_{2})\cdots c(u_{n})\ .$ 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 $R\langle A\rangle$ .