  I consider the process $\mu^+\mu^-\to e\bar{\nu}W^+$ in the case when the
effective mass of the $(e\bar\nu)$ system than the muon mass. In this case the
momentum transferred from the initial muon to the $e\bar\nu$ system (the
virtual neutrino momentum) can be both time--like and space--like. Since the
path of integration over $k^2$ goes through a pole at $k^2=0$, it gives a
divergent cross section. In the ideal case of large enough beams this
divergence disappears if the finite width $\Gamma$ of the initial muon is taken
into account. The obtained cross section corresponds to the flux of equivalent
neutrino, which coincides with that of muon (with some energy distribution). In
practice, the effect of final size of the muon beam reduces this cross section
very strong, and the effect is hardly observable.
