2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/40086We present a sum rule relating the electron energy spectrum and the hadron mass distribution in semileptonic b -> u decays close to threshold. The relation found is free from non-perturbative effects and the theoretical error is expected to be O(5%). An experimental confirmation of this prediction can provide a check of the basic assumptions at the root of the theory of the shape function.LaTeX file, 7 pages, no figures; minor changes, details on the derivation of the one-loop coefficient function addedHigh Energy Physics - Phenomenologytext