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Strongly graded generalized module in the HLZ setting

Definition

Let V be an ordinary non-super vertex algebra in the sense of def-vertex-algebra-state-field. A Möbius vertex algebra is V with an integer grading V=⊕n∈ZV(n) satisfying V(n)={v∈V:L(0)v=nv}, and a representation of sl(2) by L(-1), L(0), L(1), satisfying [L(0),L(−1)]=L(−1), [L(0),L(1)]=−L(1), and [L(−1),L(1)]=−2L(0), with L(−1)=T and, for every v∈V and j=−1,0,1, [L(j),Y(v,x)]=∑i=0j+1(j+1i)xiY(L(j−i)v,x). A conformal vertex algebra supplies these operators through its Virasoro field. A generalized V-module uses the left-module action of def-vertex-algebra-module: a lower-truncated field map with vacuum action and the module Jacobi identity. In the Möbius case it also has a generalized L(0)-eigenspace decomposition W=⊕h∈CW[h], with (L(0)−h)Nw=0 for each w∈W[h] and some N depending on w; its additional sl(2)-action is a representation with the same three Lie brackets and satisfies, for every v∈V and j=−1,0,1, [LW(j),YW(v,x)]=∑i=0j+1(j+1i)xiYW(L(j−i)v,x),∂xYW(v,x)=YW(L(−1)v,x). In the conformal case these operators are the modes of the module action of the conformal vector, and the module and Virasoro axioms supply the same identities. Write L(0)s for the diagonal operator acting by h on W[h]; L(0)−L(0)s acts locally nilpotently on each generalized weight space, with no uniform exponent required.

Let A be an abelian group. V has integer-weight and A-gradings with V=⊕n∈ZV(n)=⊕α∈AV(α). Compatibility means V(n)(α)=V(n)∩V(α) and V(α)=⊕n∈ZV(n)(α), so V=⊕n∈Z,α∈AV(n)(α). Each V(n)(α) is finite dimensional and zero for all sufficiently negative n for each fixed α, 1∈V(0)(0), vmV(β)⊂V(α+β) for v∈V(α) and m∈Z, and L(j)V(α)⊂V(α) for j=−1,0,1. In the conformal case also require ω∈V(2)(0).

For an abelian group à containing A, a generalized module W is strongly Ã-graded when W has an auxiliary decomposition W=⊕β∈A~W(β) compatible with its generalized-weight decomposition: W[h](β):=W[h]∩W(β), W(β)=⊕h∈CW[h](β), and W[h]=⊕β∈A~W[h](β). Thus W=⊕h∈C,β∈A~W[h](β). Each W[h](β) is finite dimensional, and for each fixed h,β one has W[h+k](β)=0 for all sufficiently negative integers k. In addition, vmW(β)⊂W(α+β) for v∈V(α), L(j) preserves W(β) for j=−1,0,1, and, in the Möbius case, the action is the specified sl(2)-representation above. A morphism f:W→U is a V-module map preserving each Ã-degree and generalized weight; in the Möbius case it also satisfies fLW(j)=LU(j)f for j=−1,0,1. In the conformal case these last identities follow from V-linearity because the LW(j) are modes of the action of ω.

Facts & Assumptions

Given: Work over C with the formal-variable, grading, and branch conventions in the linked definitions.

[F1]

Conformal vertex algebra of CFT type: A conformal vertex algebra of CFT type is a complex vertex algebra V in the sense of def-vertex-algebra-state-field, with a specified vector ω∈V and scalar c∈C, such that, writing Y(ω,z)=∑n∈ZL(n)z−n−2, the operators L(n) and scalar central action c satisfy the Virasoro relations of def-virasoro-algebra, and L(−1)=T. In addition, L(0) is diagonalizable, Vn:={v∈V:L(0)v=nv} gives V=⨁n≥0Vn, every Vn is finite dimensional, and V0=C1. Under this page's convention, this is a vertex operator algebra of CFT type. The grading and the CFT-type condition are part of the definition; they do not follow from the vertex-algebra axioms alone. The conformal weight of v∈Vn is n. A conformal vector, a grading, and the nonnegative grading condition are additional data beyond an ordinary vertex algebra.

[F2]

Grading-restricted generalized module: Let V be a conformal vertex algebra of CFT type and M a left V-module. Write LM(0) for the coefficient determined by YM(ω,z)=∑n∈ZLM(n)z−n−2. For h∈C, define the generalized LM(0)-eigenspace M[h]={m∈M:(LM(0)−h)Nm=0 for some integer N≥1}. The module is grading-restricted generalized if M=⨁h∈CM[h], every M[h] is finite dimensional, and the real parts of its nonzero generalized weights are bounded below. The direct sum means each vector has only finitely many generalized-weight components. This convention explicitly adds finite-dimensionality and a lower bound to the generalized-eigenspace decomposition; those conditions are not consequences of the module Jacobi identity.

[F3]

Module for an ordinary vertex algebra: Let V be a vertex algebra over C in the ordinary, non-super convention of def-vertex-algebra-state-field. A left V-module is a complex vector space M and a linear map YM(−,z):V⟶{formal fields on M},a⟼YM(a,z)=∑n∈Za(n)Mz−n−1, with the following properties: 1. Lower truncation. For every a∈V and m∈M, a(n)Mm=0 for all sufficiently large integers n, so YM(a,z)m∈M((z)) in the support convention of def-formal-distribution-on-a-vector-space. 2. Vacuum. YM(1,z)=id⁡M. 3. Module Jacobi identity. For every a,b∈V, z0−1δ ⁣(z1−z2z0)YM(a,z1)YM(b,z2)−z0−1δ ⁣(z2−z1−z0)YM(b,z2)YM(a,z1)=z2−1δ ⁣(z1−z0z2)YM(Y(a,z0)b,z2). Here δ(x)=∑r∈Zxr; the first expansion is in nonnegative powers of z2, the second in nonnegative powers of z1, and the right-hand expansion is in nonnegative powers of z0. They use the formal binomial conventions of def-formal-distribution-on-a-vector-space. The Y inside the right-hand side is the state-field map of V; it is not the action map YM. A V-module homomorphism f:M→N is a linear map such that fYM(a,z)=YN(a,z)f for every a∈V. This definition does not impose a conformal grading, finite-dimensional weight spaces, semisimplicity of L0, a right-module convention, or a twist by an automorphism. For a conformal vertex algebra, write YM(ω,z)=∑n∈ZLM(n)z−n−2 when a conformal vector ω is specified. A V-submodule of M is a linear subspace stable under every mode a(n)M. A module is simple (or irreducible) if it is nonzero and has no proper nonzero V-submodule. A module is completely reducible (or semisimple) if it is an internal direct sum of simple V-submodules; the empty direct sum is the zero module.

[F4]

Vertex algebra in the state–field convention: A (non-super) vertex algebra over C is a complex vector space V equipped with a nonzero vector 1∈V, a linear operator T:V→V, and a linear state–field map Y(−,z):V⟶(End⁡C(V))Z,a⟼Y(a,z)=∑n∈Za(n)z−n−1, where the codomain denotes all endomorphism-valued coefficient families indexed by Z. satisfying the following axioms. 1. Field and truncation. For each a,v∈V, there is an integer N(a,v) such that a(n)v=0 for all n≥N(a,v). Thus Y(a,z)v∈V((z)) in the lower-bounded support sense of def-formal-laurent-series-and-residue. 2. Vacuum and creation. Y(1,z)=id⁡V, T1=0, and Y(a,z)1=ezTa:=∑r≥0zrTrar!. This is a formal power series; no analytic convergence is asserted. 3. Translation covariance. For every a∈V, [T,Y(a,z)]=∂zY(a,z). 4. Locality. For every a,b∈V, some integer N≥0 satisfies (z−w)N[Y(a,z),Y(b,w)]=0. The coefficient a(n)b is called the nth product of the states a,b. The mode and formal-distribution conventions in def-formal-distribution-on-a-vector-space are in force. The axioms are algebraic and do not impose a grading, conformal vector, simplicity, unitarity, or analytic OPE convergence.

Proof

1.1givenF1F2F3F4algebra

If v∈V(n)(α), then L(0)L(j)v=(n−j)L(j)v by the sl(2) brackets, and L(j) preserves α. Thus L(j)V(n)(α)⊂V(n−j)(α). The fixed-α lower cutoff makes L(1)kv=0 for sufficiently large k. Every state has finite homogeneous support, so L(1) is locally nilpotent on V.

2.1givenF1F2F3F4step 1.1algebra∎

For v∈V(n), the j=0 module commutator and derivative identity give [LW(0),vm]=(n−m−1)vm. Hence (LW(0)−(h+n−m−1))Nvmw=vm(LW(0)−h)Nw, which vanishes for some N whenever w∈W[h]. Together with auxiliary-degree compatibility this gives vmW[h](β)⊂W[h+n−m−1](α+β) for v∈V(n)(α). These are the required integer and auxiliary grading compatibilities; no uniform nilpotence exponent on W is imposed.

Depends on

Dependency tree · 2 levels

Sources