2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/62475By the results of the author and Chiantini in Math.AG/0110102, on a general quintic threefold $X \subset {\mathbf P}^4$ the minimum integer $p$ for which there exists a positive dimensional family of irreducible rank $p$ vector bundles on $X$ without intermediate cohomology is at least three. In this paper we show that $p \leq 4$, by constructing series of positive dimensional families of rank 4 vector bundles on $X$ without intermediate cohomology. The general member of such family is an indecomposable bundle from the extension class $Ext^1(E,F)$, for a suitable choice of the rank 2 ACM bundles $E$ and $F$ on $X$. The existence of such bundles of rank $p = 3$ remains under question.v2: 8 pages. Title changed. One wrong example is removed. More explicit examples are given - v3: typos corrected according to referees suggestions - v.4 final version, to appear on Central European Journal of MathematicsAlgebraic Geometry14F05text