  We give an estimate of the number $N(\lambda)$ of eigenvalues $<\lambda$ for
the image under an irreducible representation of the ``sublaplacian'' on a
stratified nilpotent Lie algebra. We also give an estimate for the trace of the
heat-kernel associated with this operator. The estimates are formulated in term
of geometrical objects related to the representation under consideration. An
important particular case is the Schr\"odinger equation with polynomial
electrical and magnetical fields.
