2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/46768The near-threshold expansion of the $ππ$ amplitude is developed using the crossing-covariant independent variables. The independent threshold parameters entering the real part of the amplitude in an explicitly Lorentz-invariant way are free from restrictions of isotopic and crossing symmetries. Parameters of the expansion of the imaginary part are recovered by the perturbative unitarity relations.17 pages, Latex (uses psfig.sty), 1 Postscript figure, submitted to YaFHigh Energy Physics - Phenomenologytext