Theorem onπl2Decays and Electron-Muon Universality

We show that the coefficient of the logarithmic lepton-mass singularity in the radiative corrections of order $\alpha$ to the total ${\pi }_{l2\mathrm{}}$ decay probability is not affected by the strong interactions and can therefore be rigorously computed. The phenomenological implications of this result and its connection with electron-muon universality are briefly discussed.