Resumen: This is a personal tribute to Lance Littlejohn on the occasion of his 70th birthday. It is meant as a present to him for many years of friendship. It is not written in the “Satz-Beweis” style of Edmund Landau or even in the format of a standard mathematics paper. It is rather an invitation to a fairly new, largely unexplored, topic in the hope that Lance will read it some afternoon and enjoy it. If he cares about complete proofs he will have to wait a bit longer; we almost have them but not in time for this volume. We hope that the figures will convince him and other readers that the phenomena displayed here are interesting enough. Idioma: Inglés DOI: 10.1007/978-3-030-75425-9_11 Año: 2021 Publicado en: Operator theory 285 (2021), 197-212 ISSN: 0255-0156 Financiación: info:eu-repo/grantAgreement/ES/DGA/E48-20R Financiación: info:eu-repo/grantAgreement/ES/DGA-FEDER/E26-17R Financiación: info:eu-repo/grantAgreement/ES/MICINN-AEI-FEDER/MTM2017-89941-P Tipo y forma: Artículo (PostPrint) Área (Departamento): Área Matemática Aplicada (Dpto. Matemática Aplicada)