2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192692We describe the $ππ$ $S$-wave in $D^+ \toπ^+ π^- π^+$ decays using a unitary model for the $ππ$ Final State Interactions (FSI). The three body decay is treated as a quasi two-body process where, at the weak vertex, the D meson decays into a resonance and a pion. The weak part of the decay amplitude is evaluated using the effective weak Hamiltonian within the factorization approximation.4 pages. Contribution to the X Hadron Physics, Florianopolis-Brazil, March 26-31, 2007High Energy Physics - Phenomenologytext