Alphabeta Math
LemmaStatement: AI-adaptedProof: AI-generatedPipeline-generatedjudge pass (gpt-6-luna)
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The product basis is orthonormal and tensor dimensions multiply

Statement

Let H0,…,Hn−1 be a finite ordered list of finite-dimensional complex inner-product spaces with specified ordered orthonormal bases of sizes d0,…,dn−1, allowing dk=0. The elementary tensors of the factor bases, ordered lexicographically, form an orthonormal basis of the tensor register, and

dim⁡(H0⊗⋯⊗Hn−1)=∏k<ndk.

For the empty list the product is 1 and the tensor space is C; if any dk=0, the tensor space is zero and the product basis is empty.

Facts & Assumptions

Given: A finite ordered list of finite-dimensional complex inner-product spaces and their specified ordered orthonormal bases.

[F1]

The tensor register's pairing is the product of the factor pairings and its ordered basis is the lexicographic product basis (Ordered tensor registers, their inner product, and wire permutations).

[F2]

For two free modules, elementary tensors of basis vectors form a basis, including an empty basis in a factor (The elementary tensors of two bases form the product basis of the tensor product).

[F3]

The dimension of a tensor product of two finite-dimensional free modules is the product of their dimensions, including dimension zero (Rm⊗RRn≅Rmn with the product basis, and dim⁡F(V⊗FW)=dim⁡FV dim⁡FW).

[F4]

Finite iterated tensor products are identified across parenthesizations by the canonical maps preserving elementary tensors, and the empty product is the scalar module (Finite iterated tensor products represent multilinear maps independently of parenthesization).

Proof

1.1givenF1algebra

If n=0, the declared tensor space is C, its product basis is {1}, and its dimension is 1, the empty product. If n=1, the statement is exactly the supplied orthonormal basis of that factor.

1.2givenF1F2F4algebra

For n≥2, repeated application of [F2] and the parenthesization identifications in [F4] shows that the elementary tensors of the factor bases form a basis indexed by the Cartesian product of their index sets. By [F1], the inner product of two such basis tensors is the product of their factor inner products; it is 1 for equal tuples and 0 for unequal tuples. Therefore this basis is orthonormal.

2.1step 1.1step 1.2F2F3algebra∎

Repeated application of [F3] gives the product of the factor dimensions. If any dk=0, [F2] makes the product basis empty and both sides are zero; the empty and one-factor cases were handled in step 1.1. Thus the formula holds in every finite case.

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