We construct ‘‘almost periodic’’ unbounded domains, where a large class of elliptic spectral problems have essential spectra possessing peculiar structure: they consist of monotone, non-negative sequences of isolated points and thus have infinitely many gaps.
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Sergei A. Nazarov, Jari Taskinen, Pathology of essential spectra of elliptic problems in periodic family of beads threaded by a spoke thinning at infinity. Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Natur. 31 (2020), no. 4, pp. 939–969DOI 10.4171/RLM/921