# Filtered algebra

In mathematics, a filtered algebra is a generalization of the notion of a graded algebra. Examples appear in many branches of mathematics, especially in homological algebra and representation theory.

A filtered algebra over the field $k$ is an algebra $(A,\cdot )$ over $k$ that has an increasing sequence $\{0\}\subseteq F_{0}\subseteq F_{1}\subseteq \cdots \subseteq F_{i}\subseteq \cdots \subseteq A$ of subspaces of $A$ such that

$A=\bigcup _{i\in \mathbb {N} }F_{i}$ and that is compatible with the multiplication in the following sense:

$\forall m,n\in \mathbb {N} ,\qquad F_{m}\cdot F_{n}\subseteq F_{n+m}.$ ## Associated graded algebra

In general there is the following construction that produces a graded algebra out of a filtered algebra.

If $A$ is a filtered algebra then the associated graded algebra ${\mathcal {G}}(A)$ is defined as follows:

• As a vector space
${\mathcal {G}}(A)=\bigoplus _{n\in \mathbb {N} }G_{n}\,,$ where,

$G_{0}=F_{0},$ and
$\forall n>0,\quad G_{n}=F_{n}/F_{n-1}\,,$ • the multiplication is defined by
$(x+F_{n-1})(y+F_{m-1})=x\cdot y+F_{n+m-1}$ for all $x\in F_{n}$ and $y\in F_{m}$ . (More precisely, the multiplication map ${\mathcal {G}}(A)\times {\mathcal {G}}(A)\to {\mathcal {G}}(A)$ is combined from the maps

$(F_{n}/F_{n-1})\times (F_{m}/F_{m-1})\to F_{n+m}/F_{n+m-1},\ \ \ \ \ \left(x+F_{n-1},y+F_{m-1}\right)\mapsto x\cdot y+F_{n+m-1}$ for all $n\geq 0$ and $m\geq 0$ .)

The multiplication is well defined and endows ${\mathcal {G}}(A)$ with the structure of a graded algebra, with gradation $\{G_{n}\}_{n\in \mathbb {N} }.$ Furthermore if $A$ is associative then so is ${\mathcal {G}}(A)$ . Also if $A$ is unital, such that the unit lies in $F_{0}$ , then ${\mathcal {G}}(A)$ will be unital as well.

As algebras $A$ and ${\mathcal {G}}(A)$ are distinct (with the exception of the trivial case that $A$ is graded) but as vector spaces they are isomorphic.

## Examples

Any graded algebra graded by ℕ, for example $A=\oplus _{n\in \mathbb {N} }A_{n}$ , has a filtration given by $F_{n}=\oplus _{i=0}^{n}A_{i}$ .

An example of a filtered algebra is the Clifford algebra $\mathrm {Cliff} (V,q)$ of a vector space $V$ endowed with a quadratic form $q.$ The associated graded algebra is $\bigwedge V$ , the exterior algebra of $V.$ The symmetric algebra on the dual of an affine space is a filtered algebra of polynomials; on a vector space, one instead obtains a graded algebra.

The universal enveloping algebra of a Lie algebra ${\mathfrak {g}}$ is also naturally filtered. The PBW theorem states that the associated graded algebra is simply $\mathrm {Sym} ({\mathfrak {g}})$ .

Scalar differential operators on a manifold $M$ form a filtered algebra where the filtration is given by the degree of differential operators. The associated graded algebra is the commutative algebra of smooth functions on the cotangent bundle $T^{*}M$ which are polynomial along the fibers of the projection $\pi \colon T^{*}M\rightarrow M$ .

The group algebra of a group with a length function is a filtered algebra.

## See also

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