2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/69462In this note we further develop the idea of using a ``black box'' point of view (see our previous work) to study eigenfunctions for billiards which have rectangular components: they include the Bunimovich billiard, the Sinai billiard, and the recently popular pseudointegrable billiards6 pages, 3 figuresSpectral TheoryAnalysis of PDEstext