2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/228830This talk is dedicated to the memory of Paco Yndurain, the original speaker in the conference. After a short account of his scientific career, we briefly review our ongoing collaboration to determine precisely the $ππ$ scattering amplitude including the most recent data by means of Forward Dispersion Relations and Roy Equations. A remarkable improvement in precision over the intermediate energy region is obtained by using once-subtracted Roy Equations in addition to the standard twice-subtracted ones.Invited talk in the QCD08 14th International QCD Conference. 7-12th July 2008 Montpellier (France)High Energy Physics - Phenomenologytext