Difference between revisions of "Fatou ring"
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| − | An [[Integral domain|integral domain]] $A$ with quotient field $K$ such that if each rational function $R\in K(X)$ that has a Taylor expansion at $0$ with coefficients in $A$, has a unitary and irreducible representation with coefficients in $A$; that is, for each | + | An [[Integral domain|integral domain]] $A$ with quotient field $K$ such that if each rational function $R\in K(X)$ that has a Taylor expansion at $0$ with coefficients in $A$, has a unitary and irreducible representation with coefficients in $A$; that is, for each $R\in K(X)\cap A[[X]]$ there are $P,Q\in A[X]$ such that $R=P/Q$, $Q(0)=1$ and $P$ and $Q$ are relatively prime in $K[X]$. Fatou's lemma [[#References|[a1]]] states that $\mathbb{Z}$ is a Fatou ring. |
| − | Equivalently, | + | Equivalently, $A$ is a Fatou ring means that if a sequence $\{ a_n\}_{n\in\mathbb{N}}$ of elements of $A$ satisfies a linear recursion formula |
| − | + | $$a_{n+s}+q_1a_{n+1}+\cdots+q_sa_n=0\textrm{ for }n\geq 0,$$ | |
| − | where | + | where $q_1,\hdots,q_s\in K$ and $s$ is as small as possible, then $q_1,\hdots,q_s\in A$. |
| − | If | + | If $A$ is a Fatou ring, then its quotient field $K$ is a [[Fatou extension|Fatou extension]] of $A$, but the converse does not hold. This is the reason why Fatou rings are sometimes called strong Fatou rings, while the domains $A$ such that $K$ is a Fatou extension of $A$ are called weak Fatou rings (in this latter case, every $R\in K(X)\cap A[[X]]$ has a unitary (not necessarily irreducible) representation with coefficients in $A$). |
| − | The coefficients of the unitary and irreducible representation of every element of | + | The coefficients of the unitary and irreducible representation of every element of $K(X)\cap A((X))$ are almost integral over $A$ (see [[Fatou extension|Fatou extension]]). An integral domain $A$ is a Fatou ring if and only if every element of $K$ which is almost integral over $A$ belongs to $A$ [[#References|[a2]]]; such domains are said to be completely integrally closed. For instance, a Noetherian domain (cf. also [[Noetherian ring|Noetherian ring]]) is completely integrally closed if and only if it is integrally closed (cf. also [[Integral domain|Integral domain]]). The rings of integers of number fields are completely integrally closed, and hence, Fatou rings. |
The notion may be extended by considering [[Formal power series|formal power series]] in non-commuting variables. The characterization of this generalized property is still (1998) an open question. | The notion may be extended by considering [[Formal power series|formal power series]] in non-commuting variables. The characterization of this generalized property is still (1998) an open question. | ||
Revision as of 18:51, 19 April 2012
An integral domain $A$ with quotient field $K$ such that if each rational function $R\in K(X)$ that has a Taylor expansion at $0$ with coefficients in $A$, has a unitary and irreducible representation with coefficients in $A$; that is, for each $R\in K(X)\cap A[[X]]$ there are $P,Q\in A[X]$ such that $R=P/Q$, $Q(0)=1$ and $P$ and $Q$ are relatively prime in $K[X]$. Fatou's lemma [a1] states that $\mathbb{Z}$ is a Fatou ring.
Equivalently, $A$ is a Fatou ring means that if a sequence $\{ a_n\}_{n\in\mathbb{N}}$ of elements of $A$ satisfies a linear recursion formula
$$a_{n+s}+q_1a_{n+1}+\cdots+q_sa_n=0\textrm{ for }n\geq 0,$$
where $q_1,\hdots,q_s\in K$ and $s$ is as small as possible, then $q_1,\hdots,q_s\in A$.
If $A$ is a Fatou ring, then its quotient field $K$ is a Fatou extension of $A$, but the converse does not hold. This is the reason why Fatou rings are sometimes called strong Fatou rings, while the domains $A$ such that $K$ is a Fatou extension of $A$ are called weak Fatou rings (in this latter case, every $R\in K(X)\cap A[[X]]$ has a unitary (not necessarily irreducible) representation with coefficients in $A$).
The coefficients of the unitary and irreducible representation of every element of $K(X)\cap A((X))$ are almost integral over $A$ (see Fatou extension). An integral domain $A$ is a Fatou ring if and only if every element of $K$ which is almost integral over $A$ belongs to $A$ [a2]; such domains are said to be completely integrally closed. For instance, a Noetherian domain (cf. also Noetherian ring) is completely integrally closed if and only if it is integrally closed (cf. also Integral domain). The rings of integers of number fields are completely integrally closed, and hence, Fatou rings.
The notion may be extended by considering formal power series in non-commuting variables. The characterization of this generalized property is still (1998) an open question.
References
| [a1] | P. Fatou, "Sur les séries entières à coefficients entiers" C.R. Acad. Sci. Paris Ser. A , 138 (1904) pp. 342–344 |
| [a2] | S. Eilenberg, "Automata, languages and machines" , A , Acad. Press (1974) |
How to Cite This Entry:
Fatou ring. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Fatou_ring&oldid=24793
Fatou ring. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Fatou_ring&oldid=24793
This article was adapted from an original article by Jean-Luc Chabert (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article