Converse characterization of the essential approximate point spectrum for elliptic and double parabolic Blaschke products

Let BB be an elliptic or double parabolic Blaschke product of degree d2d \geq 2, and let TT be the weighted composition operator on the disc algebra associated with BB and a weight ww. Consider the set of moduli of scalars whose circles lie in the upper semi-Fredholm spectrum:

{λ:λ\mathdsTσusf(T)}.\{\lvert\lambda\rvert: \lambda \mathds{T} \subset \sigma_{usf}(T)\}.

Converse characterization conjecture. The set {λ:λ\mathdsTσusf(T)}\{\lvert\lambda\rvert: \lambda \mathds{T} \subset \sigma_{usf}(T)\} is at most countable and its only accumulation point, if any, is 00.

The surrounding discussion presents this as the converse to the preceding theorem for finite Blaschke products. The supplied text does not establish the converse, so its resolution remains open.

Sources & referencesView supporting material

Primary source

Arkady Kitover and Mehmet Orhon, “Spectrum of weighted composition operators. Part XI. The essential spectra of some weighted composition operators on the disc algebra”, arXiv:2503.17516 (2025).

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