# nLab Laurent series

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Idea

Laurent series generalize power series by allowing both positive and negative powers. In particular, Laurent series with complex coefficients generalize Taylor series of analytic functions to meromorphic functions. A Laurent series for a meromorphic function $f(z)$ at finite $z\in\mathbb{C}$ has the form

$f(z) = \sum_{n=k}^{\infty}f_n z^n,$

where $k$ is merely constrained to be finite and is often negative.

Or, in some contexts one wants to take $k = -\infty$. Here such a formal sum (with powers extending infinitely in both directions) is suggestive notation for an element belonging to the dual $\prod_{n \in \mathbb{Z}} R \cdot z^n$ of a ring $\oplus_{n \in \mathbb{Z}} R \cdot z^n$ (see Laurent polynomials below). In such contexts, Laurent series can be likened to distributions, i.e., functionals on the algebra of functions $\mathbb{Z} \to R$ with compact support.

## Definition

###### Definition

A Laurent series in one variable $z$ over a commutative unital ring $k$ is a doubly infinite series

$f(z) = \sum_{n=-\infty}^{\infty} f_n z^n ,$

where $f_n\in k$. Equivalently: a Laurent series is a function $\mathbb{Z} \to k: n \mapsto f_n$. The $k$-module of Laurent series is denoted $k[ [z, z^{-1}] ]$.

A Laurent polynomial is a Laurent series for which all but finitely many $f_n$ are zero. Laurent polynomials form a ring which may be described as $k[z, z^{-1}]$ or abstractly as $k[x, y]/(x y - 1)$. Observe that Laurent series in the generality discussed here do not analogously form a ring: the obvious definition of the coefficients of the product $h(z) = f(z)g(z)$ of two Laurent series $f(z), g(z)$,

$h_n = \sum_{k \in \mathbb{Z}} f_k g_{n-k},$

doesn’t make sense in general (although it could sometimes make sense in topological contexts where some such infinite sums can converge, as in the case $k = \mathbb{C}$).

###### Remark

The $k$-vector space of Laurent series does however form a module over the ring of Laurent polynomials, i.e., if $f(z) \in k[z, z^{-1}]$ and $g(z)$ is a Laurent series, then the product $h(z) = f(z)g(z)$ as defined above always makes sense.

In general, questions of convergence are treated as separate issues. In complex analysis, the Laurent series $\sum_{n \in \mathbb{Z}} a_n z^n$ describes a meromorphic function in a neighborhood around the point $z = 0$ (possibly with a pole there) if all but finitely many negatively indexed terms are zero. Similarly, series of the form $\sum_{n = -N}^{\infty} a_n (z-a)^n$ describe meromorphic functions in a neighborhood of $z=a$ with poles of order at most $N$.

On the other hand, in algebra one often hears of the ring of formal Laurent series. Here, the presence of the word “ring” signifies that we are restricting the coefficients so that multiplication makes sense. Thus,

###### Definition

The ring of formal Laurent series over a commutative ring $A$ in an indeterminate $x$ consists of Laurent series $\sum_{n \in \mathbb{Z}} f_n z^n$, with $f_n \in A$ but where all but finitely many $f_n$ for $n \lt 0$ vanish.

Multiplication defined as above clearly makes sense. If $A$ is a field $k$, then this ring is usually denoted $k((x))$ and is in fact a field; indeed it is the field of fractions of the ring $k[ [x] ]$ of formal power series, where $k[ [x] ]$ is often viewed as a discrete valuation ring.

###### Remark

It would perhaps be clearer if we used the term “restricted Laurent series” to cover the Laurent series considered in Definition 2, and let “Laurent series” be the term that covers doubly infinite series.

## Properties

### Laurent series as distributions

Another point of view on Laurent series is given in the following alternative definition.

###### Definition

A Laurent series over $k$ is a $k$-linear functional

$\phi: k[z, z^{-1}] \to k$

on the algebra of Laurent polynomials.

The associated series is $\sum_{n \in \mathbb{Z}} \phi(z^n)z^n$. But here we emphasize the point of view that Laurent series have a distribution-like character, with Laurent polynomials being considered a space of functions $\mathbb{Z} \to k$ with compact (finite) support, via the evident inclusion

$k[z, z^{-1}] \cong \oplus_{n \in \mathbb{Z}} k \cdot z^n \hookrightarrow \prod_{n \in \mathbb{Z}} k \cdot z^n = k^{\mathbb{Z}}.$

Of particular interest as a Laurent series is the formal Dirac distribution,

$\delta(z) = \sum_{n \in \mathbb{Z}} z^n,$

which intuitively is like the Fourier transform $\sum_{n = -\infty}^\infty e^{i n x}$ of the pointwise multiplicative identity $\mathbf{1}$ given by $\mathbf{1}(n) = 1$ for all $n$. It has the following property:

• For $f(z)$ a Laurent polynomial, $f(z)\delta(z) = f(1)\delta(z)$.

Indeed, notice that $\delta(z)$ is the distribution $p \mapsto p(1)$ which evaluates a Laurent polynomial at the multiplicative identity $z=1$.

More generally, for $a \in k^\ast$ an invertible unit, the series $\delta(a z) = \sum_{n \in \mathbb{Z}} a^n z^z$ satisfies

• $f(z)\delta(a z) = f(a^{-1})\delta(a z)$.

Just as it doesn’t make sense in general to multiply distributions (at least not without heavy qualifications, as in the theory of Colombeau), we cannot make sense of expressions like $\delta(z)^2$. However, one can meaningfully work with derivatives of distributions, so we have for example

$\delta'(z) = \sum_{n \in \mathbb{Z}} n z^n$

which has the property

• $f(z)\delta'(z) = f(1)\delta'(z) - f'(1)\delta(z)$.

Indeed, there is a formal calculus of the Dirac distribution that plays an important role in the theory of vertex operator algebras. See for example the treatment in Frenkel, Lepowsky, Meurman.

One can also work with convolution products $h(z) = (f \ast g)(z)$, defined by multiplying coefficients degree-wise:

$h(z) = \sum_{n \in \mathbb{Z}} f_n g_n z^n.$

### Algebraic closure

###### Theorem

If $K$ is algebraically closed and has characteristic 0, then the algebraic closure of the field of (restricted) Laurent series $K((x))$ over $K$ is the field of Puiseux series over $K$.

Here “restricted” refers to Remark 2. See Puiseux series for more details on this result.

## References

• Igor Frenkel, James Lepowsky, and Arne Meurman, Vertex Operator Algebras and the Monster, Volume 134 in Pure and Applied Mathematics, Academic Press 1988.

For discussion of products of distributions, see

• J.F. Colombeau, Multiplication of distributions, Bull. Amer. Math. Soc. (N.S.) Volume 23, Number 2 (1990), 251-268. (web)
Revised on September 20, 2013 21:32:38 by Todd Trimble (67.81.95.215)