2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/126057We show that, contrarily to the widespread belief, in quantum mechanics repeatable measurements are not necessarily described by orthogonal projectors--the customary paradigm of "observable". Nonorthogonal repeatability, however, occurs only for infinite dimensions. We also show that when a non orthogonal repeatable measurement is performed, the measured system retains some "memory" of the number of times that the measurement has been performed.4 pages, 1 figure, revtex4, minor changesQuantum Physicstext