2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/45502A kind of asymptotic expansion for calculation of the (para)positronium decay rate is formulated. The method is completely relativistic and gauge invariant. The expansion goes over the inverse square of relative momentum $k$ of two photons in Euclidean region and the actual width is determined at $k^2=-m^2$, $m$ being the electron mass.Latex fileHigh Energy Physics - Phenomenologytext