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Positive weights characterize the specified Hermitian form
Statement
For the semisimple closed model determined by the stated idempotent data, define when , and set . The form is a Hermitian positive-definite inner product exactly when every weight is a positive real number. The algebraic closed oriented TQFT exists for every nonzero complex weight; positivity is an additional condition for this specified Hilbert-space interpretation.
Statement references
Facts & Assumptions
Given: The semisimple model and its idempotent basis from Semisimple closed two-dimensional TQFT data. The involution is conjugate-linear and fixes each . We use the convention that a sesquilinear form is conjugate-linear in its first argument and linear in its second; it is Hermitian when , and positive definite when for every .
Semisimple closed two-dimensional TQFT data: , , and .
Semisimple closed two-dimensional TQFT data: every nonzero complex weight gives the stated algebraic closed oriented TQFT through the closed classification; no positivity hypothesis occurs there.
Proof
If and , then , so .
If every is positive real, the formula in step 1.1 is Hermitian and for each nonzero , because at least one coefficient is nonzero.
Conversely, if is Hermitian and positive definite, then is real by Hermitian symmetry and strictly positive by positive definiteness for every nonzero basis vector . Thus all weights are positive real, proving the equivalence.
The classification used in [F2] assumes only the nonzero Frobenius weights, so the algebraic closed theory remains defined when the extra positivity condition fails; this separates the categorical model from the specified Hilbert-space interpretation.
Form matrix
For the basis , the associated sesquilinear form has diagonal matrix . Positivity is a property of this explicit matrix and does not follow from the existence of the oriented closed TQFT.
Depends on
Dependency tree · 2 levels
Sources
- Joachim Kock, Frobenius algebras and 2D topological quantum field theories (short version)
- Joachim Kock, Frobenius Algebras and 2D Topological Quantum Field Theories (Cambridge book excerpt)
- Aaron D. Lauda and Hendryk Pfeiffer, Open-closed strings: Two-dimensional extended TQFTs and Frobenius algebras, arXiv:math/0510664v3