2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/65964We consider a normalized basis in a Banach space with the following property: any normalized block sequence of the basis has a subsequence equivalent to the basis. We show that under uniformity or other natural assumptions, a basis with this property is equivalent to the unit vector basis of $c_0$ or $\ell_p$. We also address an analogous problem for spreading models.14 pagesFunctional Analysis46B20; 46B15text