Some analytic properties of three-pion decay amplitudes

It is shown for all graphs of the ‘strip approximation’ that the K ->3n decay amplitude obeys a Mandelstam double-dispersion relation. The double spectral function turns out to be complex and covers the whole physical domain, so that the Cini-Fubini approximation is no longer valid. Essential singularities are found on the border of the physical sheet. Double-dispersion relations in one energy and in the K mass are also derived.