  A semiclassical approach for calculating shell effects, that has been used in
atomic and plasma physics, is applied to describe the electronic supershells in
metal clusters. Using the spherical jellium model we give the analytical
expression for the oscillating part of the binding energy of electrons as an
explicit sum of contributions from supershells with quantum numbers 2n_r+l,
3n_r+l, 4n_r+l, ... This expression is written in terms of the classical
characteristics of the motion of an electron with the Fermi energy in a
self-consistent potential. The conditions under which a new supershell appears
and the relative contribution of this shell are studied as a function of the
cluster size and form of the potential. Specific calculations are performed for
a square well.
