2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/147077The following corollary has been added: for general tetragonal curve $C$ of genus $g\ge 9$ the homogeneous coordinate ring of $C$ defined by the line bundle $K(-T)$, where $K$ is the canonical class, $T$ is the tetragonal series, is Koszul. Also some misprints are corrected.17 pages, LatexAlgebraic Geometrytext