Experimental search for the neutrino decay nu 3--> nu j+e++e- and limits on neutrino mixing.

An experiment searching for the neutrino decay {nu}{sub 3}{r_arrow}{nu}{sub {ital j}}+{ital e}{sup +}+{ital e}{sup {minus}} has been performed at the nuclear power reactor of Bugey. The experimental setup and the data analysis are presented. No evidence for this decay has been found and stringent limits on the coupling {vert_bar}{ital U}{sub {ital e}3}{vert_bar}{sup 2} of a massive neutrino in the range of 1 MeV to 10 MeV to an electron are derived. Consequences for the existence of a massive {nu}{sub {tau}} are briefly discussed in the context of astrophysical and cosmological arguments.