2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/195017We construct stable bundle extensions on elliptically fibered Calabi-Yau threefolds. We show that these bundles can solve the topological anomaly constraint in heterotic string theory without the need of invoking background fivebranes.18 pages, minor changesAlgebraic GeometryHigh Energy Physics - Theory14J60, 14J32, 81T30, 83E30text