2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/99361We discuss differences between the exact S-matrix for scattering on serial structures and a known factorized expression constructed of single-element S-matrices. As an illustration, we use an exactly solvable model of a quantum wire with two point impurities.A LaTeX file, 10 pages, with one ps figure; to appear in Czech.J.PhysMesoscale and Nanoscale PhysicsQuantum Physicstext