2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/108953Let X be a K3 surface with a primitive ample divisor H, and let $β=2[H]\in H_2(X, \mathbf Z)$. We calculate the Gromov-Witten type invariants $n_β$ by virtue of Euler numbers of some moduli spaces of stable sheaves. Eventually, it verifies Yau-Zaslow formula in the non primitive class $β$.15 pages, added references, to appear in Asian J. MathAlgebraic Geometrytext