Strong modular Zauner's conjecture for Hilbert C*-modules

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Let A\mathcal{A} be a unital C*-algebra with the invariant basis number property or let A\mathcal{A} be a W*-algebra. For d∈Nd\in\mathbb{N}, consider unit inner product frames in the standard module Ad\mathcal{A}^d, with 1d+1\frac{1}{d+1}-equiangularity meaning that mutual inner-product products equal 1d+1\frac{1}{d+1}. Strong modular Zauner's conjecture. For every d∈Nd\in\mathbb{N}, there exists a 1d+1\frac{1}{d+1}-equiangular unit inner product frame {τj}j=1d2\{\tau_j\}_{j=1}^{d^2} for Ad\mathcal{A}^d, equivalently a tight frame with unit diagonal inner products and

⟨τj,τk⟩⟨τk,τj⟩=1d+1,1≤j,k≤d2, j≠k.\langle\tau_j,\tau_k\rangle\langle\tau_k,\tau_j\rangle=\frac{1}{d+1},\qquad 1\leq j,k\leq d^2,\ j\neq k.

The extra algebraic hypotheses define the strong form of the modular extension of Zauner's conjecture. The supplied text does not state whether this form is open, solved, or refuted.

References

Primary source

K. Mahesh Krishna, “Modular Welch Bounds with Applications”, arXiv:2201.00319 (2022).

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