2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/31343It has long been an open question whether the formula XCB = EpEEEpqErqr is, with the rules of substitution and detachment, a single axiom for the classical equivalential calculus. This paper answers that question affirmatively, thus completing a search for all such eleven-symbol single axioms that began seventy years ago.6 pages, no figuresLogic in Computer ScienceArtificial IntelligenceF.4.1; I.2.3text