2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/70141Pattern avoidance classes of permutations that cannot be expressed as unions of proper subclasses can be described as the set of subpermutations of a single bijection. In the case that this bijection is a permutation of the natural numbers a structure theorem is given. The structure theorem shows that the class is almost closed under direct sums or has a rational generating function.18 pages, 4 figures (all in-line)Combinatorics05A05text