This article presents a new proof of a theorem concerning bounds of the spectrum of the product of unitary operators and a generalization for differentiable curves of this theorem. The proofs involve metric geometric arguments in the group of unitary operators and the sphere where these operators act.
Commutation Relations for Unitary Operators II
Quadratic forms in unitary operators
Truncated moment problems for $J$-self-adjoint, $J$-skew-self-adjoint and $J$-unitary operators
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