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In mathematics, the Zhu algebra and the closely related C₂-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 C₂-algebra.

Definitions

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

The algebra ${\displaystyle A(V):=V/O(V)}$ with the binary operation induced by ${\displaystyle *}$ is an associative algebra called the Zhu algebra of ${\displaystyle V}$.

The algebra ${\displaystyle R_{V}:=V/C_{2}(V)}$ with multiplication ${\displaystyle a\cdot b=a_{-1}b\mod C_{2}(V)}$ is called the C₂-algebra of ${\displaystyle V}$.

Main properties

• The multiplication of the C₂-algebra is commutative and the additional binary operation ${\displaystyle \{a,b\}=a_{0}b\mod C_{2}(V)}$ is a Poisson bracket on ${\displaystyle R_{V}}$which gives the C₂-algebra the structure of a Poisson algebra.
• (Zhu's C₂-cofiniteness condition) If ${\displaystyle R_{V}}$is finite dimensional then ${\displaystyle V}$ is said to be C₂-cofinite. There are two main representation theoretic properties related to C₂-cofiniteness. A vertex operator algebra ${\displaystyle V}$ 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 C₂-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 C₂-cofinite but not rational. Various weaker versions of this conjecture are known, including that regularity implies C₂-cofiniteness and that for C₂-cofinite ${\displaystyle V}$ 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 ${\displaystyle V}$ induces a filtration ${\displaystyle A(V)=\bigcup _{p\geq 0}A_{p}(V)}$ where ${\displaystyle A_{p}(V)=\operatorname {im} (\oplus _{j=0}^{p}V_{p}\to A(V))}$so that ${\displaystyle A_{p}(V)\ast A_{q}(V)\subset A_{p+q}(V).}$ There is a surjective morphism of Poisson algebras ${\displaystyle R_{V}\to \operatorname {gr} (A(V))}$.

Associated variety

Because the C₂-algebra ${\displaystyle R_{V}}$ is a commutative algebra it may be studied using the language of algebraic geometry. The associated scheme ${\displaystyle {\widetilde {X}}_{V}}$ and associated variety ${\displaystyle X_{V}}$ of ${\displaystyle V}$ are defined to be $${\displaystyle {\widetilde {X}}_{V}:=\operatorname {Spec} (R_{V}),~~~X_{V}:=({\widetilde {X}}_{V})_{\mathrm {red} }}$$which are an affine scheme an affine algebraic variety respectively. Moreover, since ${\displaystyle L(-1)}$ acts as a derivation on ${\displaystyle R_{V}}$ there is an action of ${\displaystyle \mathbb {C} ^{\ast }}$ on the associated scheme making ${\displaystyle {\widetilde {X}}_{V}}$ a conical Poisson scheme and ${\displaystyle X_{V}}$ a conical Poisson variety. In this language, C₂-cofiniteness is equivalent to the property that ${\displaystyle X_{V}}$ is a point.

Example: If ${\displaystyle W^{k}({\widehat {\mathfrak {g}}},f)}$ is the affine W-algebra associated to affine Lie algebra ${\displaystyle {\widehat {\mathfrak {g}}}}$ at level ${\displaystyle k}$ and nilpotent element ${\displaystyle f}$ then ${\displaystyle {\widetilde {X}}_{W^{k}({\widehat {\mathfrak {g}}},f)}={\mathcal {S}}_{f}}$is the Slodowy slice through ${\displaystyle f}$.

References

1. ^ Zhu, Yongchang (1996). "Modular invariance of characters of vertex operator algebras". Journal of the American Mathematical Society. 9 (1): 237–302. doi:10.1090/s0894-0347-96-00182-8. ISSN 0894-0347.
2. ^ Li, Haisheng (1999). "Some Finiteness Properties of Regular Vertex Operator Algebras". Journal of Algebra. 212 (2): 495–514. arXiv:math/9807077. doi:10.1006/jabr.1998.7654. ISSN 0021-8693. S2CID 16072357.
3. Dong, Chongying; Li, Haisheng; Mason, Geoffrey (1997). "Regularity of Rational Vertex Operator Algebras". Advances in Mathematics. 132 (1): 148–166. arXiv:q-alg/9508018. doi:10.1006/aima.1997.1681. ISSN 0001-8708. S2CID 14942843.
4. Adamović, Dražen; Milas, Antun (2008-04-01). "On the triplet vertex algebra W(p)". Advances in Mathematics. 217 (6): 2664–2699. doi:10.1016/j.aim.2007.11.012. ISSN 0001-8708.
5. Abe, Toshiyuki; Buhl, Geoffrey; Dong, Chongying (2003-12-15). "Rationality, regularity, and 𝐶₂-cofiniteness". Transactions of the American Mathematical Society. 356 (8): 3391–3402. doi:10.1090/s0002-9947-03-03413-5. ISSN 0002-9947.
6. Arakawa, Tomoyuki; Lam, Ching Hung; Yamada, Hiromichi (2014). "Zhu's algebra, C2-algebra and C2-cofiniteness of parafermion vertex operator algebras". Advances in Mathematics. 264: 261–295. doi:10.1016/j.aim.2014.07.021. ISSN 0001-8708. S2CID 119121685.
7. Arakawa, Tomoyuki (2010-11-20). "A remark on the C 2-cofiniteness condition on vertex algebras". Mathematische Zeitschrift. 270 (1–2): 559–575. arXiv:1004.1492. doi:10.1007/s00209-010-0812-4. ISSN 0025-5874. S2CID 253711685.
8. Arakawa, T. (2015-02-19). "Associated Varieties of Modules Over Kac-Moody Algebras and C2-Cofiniteness of W-Algebras". International Mathematics Research Notices. arXiv:1004.1554. doi:10.1093/imrn/rnu277. ISSN 1073-7928.

• Algebras
• Algebraic geometry