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Compatibility subspace yields the P(z)-tensor product
Statement
Let C be a full category of grading-restricted generalized V-modules and W1,W2∈C. If P(z) W2 with its induced P(z)-dual module action and its contragredient S' are both objects of C, then its contragredient (W1 P(z) W2)' together with the dual of the natural inclusion W1 P(z) W2↪(W1⊗W2)* is the P(z)-tensor product of W1,W2 in C.
Facts & Assumptions
Given: Work over C with the formal-variable, grading, and branch conventions in the linked definitions.
P(z)-tensor product in a module category: For a strongly A-graded Möbius or conformal vertex algebra V, an abelian group à containing A, z∈C×, and a specified full category C of strongly Ã-graded generalized V-modules with grading-preserving Möbius-module morphisms, a P(z)-tensor product of W1,W2∈C is an object T∈C and a P(z)-intertwining map ι:W1⊗W2→T̄ such that for every X∈C, composition with ι gives a natural bijection . Existence is an additional theorem or hypothesis, not part of the definition.
P(z)-compatibility subspace and local grading restriction defines the transported field by the displayed residue formula and defines by lower truncation of this field together with the transported compatibility identity. Its P(z)-local grading restriction requires λ to be a finite sum of generalized -eigenvectors, each homogeneous for ; the generated doubly graded subspace must have finite-dimensional components and be lower truncated in each fixed integer weight string. For each P(z)-map with target , the transpose defines an image; the definition of sums these images and states that transposed Jacobi and grading put each one in the stated subspace.
Strongly graded contragredient duality for Möbius and conformal modules proves the opposite-field restricted dual, componentwise evaluation , and functorial restricted transposition for strongly graded generalized modules over strongly graded Möbius or conformal V, preserving the auxiliary and generalized-weight gradings and all three Möbius operators. It imposes no CFT-type hypothesis. Use the ambient strong gradings and morphisms stipulated in F1 and F2. Each global generalized-weight space here is finite dimensional, so only finitely many auxiliary degrees occur at a fixed weight; the doubly restricted dual therefore agrees with used below. Its weight support and global grading restriction are the same as those of M.
P(z)-intertwining map: Let V be strongly A-graded Möbius or conformal, let à contain A, and let W1,W2,W3 be strongly Ã-graded generalized V-modules; fix z∈C×. Retain the ambient strong gradings from F1 and F2. The morphisms preserve those gradings and commute with the three Möbius operators; no conformal vector, CFT-type condition or trivial auxiliary-group specialization is required. A P(z)-intertwining map of type (W3;W1,W2) is a linear map whose Ã-degrees add, for which for all sufficiently large integers m for each fixed h,w1,w2, and which satisfies the P(z)-Jacobi identity and, for , I((L(j-i)w1)⊗w2). There is no L(-1)-derivative axiom for I. Formal variables x0,x1 are independent; z is a fixed complex number.
Proof
Write and . For define in by evaluation on each finite-dimensional : its pairing with is . The value generally belongs to the completion, not to . Each is a finite sum of vectors in images by the definition of . Grading compatibility follows from the grading of each . Lower truncation follows uniformly because is grading restricted: if its real weights are bounded below by , then for every integer , independently of the vector used to test the component. The compatibility identity for is exactly the transpose of P(z)-Jacobi for , and the transported identities transpose to its three required bracket identities. Thus is a P(z)-map. The assumption is needed here; fullness alone does not imply it.
For a P(z)-map with , the map , , has image in by its defining sum. Transposed P(z)-Jacobi makes it a module homomorphism for the induced action on . The transposed three Möbius identities also make it intertwine the transported Möbius operators, and the degree-addition condition preserves auxiliary degree; in particular it preserves generalized weights. Its restricted transpose is therefore a grading-preserving Möbius-module map by F3. Identify with using F3; since is full and , the resulting map is a morphism of . Evaluation of pairings gives .
Conversely, if has this property, transpose it and use the evaluation identifications , . For every , its transpose must be the vector in with values for all . The inclusion is injective, so this vector is uniquely . Thus is unique. The pairing formula commutes with postcomposition, which gives naturality and the representing property.