2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/230236Once and twice subtracted crossing symmetric dispersion relations applied to $ππ\to ππ$ scattering data are analyzed and compared. Both sets of dispersion relations can be used to test the $ππ$ amplitudes in low partial waves up to about 1 GeV. We show how once subtracted dispersion relations can provide stronger constraints for $ππ$ amplitudes than twice subtracted ones in the 400 to 1100 MeV range, given the same experimental input.6 pages, to appear in proceedings of ExcitedQCD 09 workshop, 8-14 February 2009, Zakopane (Poland)High Energy Physics - Phenomenologytext