# Zhu algebra

In mathematics, the Zhu algebra and the closely related C$_2$-algebra, introduced by Yongchang Zhu in his PhD thesis, are two associative algebras canonically constructed from a given vertex operator algebra. Many important representation theoretic properties of the vertex algebra are logically related to properties of its Zhu algebra or C$_2$-algebra.

## Definitions

Let $V=\bigoplus _{n\geq 0}V_{(n)}$ be a graded vertex operator algebra with $V_{(0)}=\mathbb {C} \mathbf {1}$ and let $Y(a,z)=\sum _{n\in \mathbb {Z} }a_{n}z^{-n-1}$ be the vertex operator associated to $a\in V.$ Define $C_{2}(V)\subset V$to be the subspace spanned by elements of the form $a_{-2}b$ for $a,b\in V.$ An element $a\in V$ is homogeneous with $\operatorname {wt} a=n$ if $a\in V_{(n)}.$ There are two binary operations on $V$defined by$a*b=\sum _{i\geq 0}{\binom {\operatorname {wt} a}{i}}a_{i-1}b,~~~~~a\circ b=\sum _{i\geq 0}{\binom {\operatorname {wt} a}{i}}a_{i-2}b$for homogeneous elements and extended linearly to all of $V$. Define $O(V)\subset V$to be the span of all elements $a\circ b$.
The algebra $A(V):=V/O(V)$ with the binary operation induced by $*$ is an associative algebra called the Zhu algebra of $V$.
The algebra $R_{V}:=V/C_{2}(V)$ with multiplication $a\cdot b=a_{-1}b\mod C_{2}(V)$ is called the C$_2$-algebra of $V$.

## Main properties

* The multiplication of the C$_2$-algebra is commutative and the additional binary operation $\{a,b\}=a_{0}b\mod C_{2}(V)$ is a Poisson bracket on $R_{V}$which gives the C$_2$-algebra the structure of a Poisson algebra.
* (Zhu's C$_2$-cofiniteness condition) If $R_{V}$is finite dimensional then $V$ is said to be C$_2$-cofinite. There are two main representation theoretic properties related to C$_2$-cofiniteness. A vertex operator algebra $V$ is rational if the category of admissible modules is semisimple and there are only finitely many irreducibles. It was conjectured that rationality is equivalent to C$_2$-cofiniteness and a stronger condition regularity, however this was disproved in 2007 by Adamovic and Milas who showed that the triplet vertex operator algebra is C$_2$-cofinite but not rational. Various weaker versions of this conjecture are known, including that regularity implies C$_2$-cofiniteness and that for C$_2$-cofinite $V$ the conditions of rationality and regularity are equivalent. This conjecture is a vertex algebras analogue of Cartan's criterion for semisimplicity in the theory of Lie algebras because it relates a structural property of the algebra to the semisimplicity of its representation category.
* The grading on $V$ induces a filtration $A(V)=\bigcup _{p\geq 0}A_{p}(V)$ where $A_{p}(V)=\operatorname {im} (\oplus _{j=0}^{p}V_{p}\to A(V))$so that $A_{p}(V)\ast A_{q}(V)\subset A_{p+q}(V).$ There is a surjective morphism of Poisson algebras $R_{V}\to \operatorname {gr} (A(V))$.

## Associated variety

Because the C$_2$-algebra $R_{V}$ is a commutative algebra it may be studied using the language of algebraic geometry. The associated scheme ${\widetilde {X}}_{V}$ and associated variety $X_{V}$ of $V$ are defined to be ${\widetilde {X}}_{V}:=\operatorname {Spec} (R_{V}),~~~X_{V}:=({\widetilde {X}}_{V})_{\mathrm {red} }$which are an affine scheme and an affine algebraic variety respectively. Moreover, since $L(-1)$ acts as a derivation on $R_{V}$ there is an action of $\mathbb {C} ^{\ast }$ on the associated scheme making ${\widetilde {X}}_{V}$ a conical Poisson scheme and $X_{V}$ a conical Poisson variety. In this language, C$_2$-cofiniteness is equivalent to the property that $X_{V}$ is a point.
Example: If $W^{k}({\widehat {\mathfrak {g}}},f)$ is the affine W-algebra associated to affine Lie algebra ${\widehat {\mathfrak {g}}}$ at level $k$ and nilpotent element $f$ then ${\widetilde {X}}_{W^{k}({\widehat {\mathfrak {g}}},f)}={\mathcal {S}}_{f}$is the Slodowy slice through $f$.