  Low energy neutrinoless lepton flavor violating (LFV) processes are studied
in an extension of the Standard Model (SM) by heavy SU(2)xU(1) singlet Dirac
neutrinos. An upper bound procedure is elaborated for the evaluation of
amplitudes. A comment on the extraction of heavy neutrino mixings from
astrophysical observations is given. For processes not treated in the applied
model the formalism not treated in the applied model the formalism for
evaluating the branching ratios (BRs) is presented. The processes previously
studied in the model are carefully examined. Some of the previous results are
improved. Special attention is paid to the structure of the amplitudes and BRs
as well as to the relations between BRs of different LFV processes. Numerical
analysis of the BRs is done. The decoupling of heavy neutrinos is discussed and
it is explicitely shown that the very heavy neutrinos decouple when the upper
bound procedure is applied. The upper limits of the BRs are compared with the
current experimental upper bounds and the processes interesting for the search
for LFV are proposed. The LFV decays are shown to be unsuitable for finding
upper bounds on "diagonal" LFV parameters. The B-meson LFV processes are
suggested for the search of LFV in future B-factories.
