  By use of the spectral density approach the influence of the lattice
structure on the possibility of ferromagnetism in the single band Hubbard model
is investigated. The d=\infty hypercubic lattice does not show magnetic phase
transitions of second order irrespective of the strength of the Coulomb
coupling. However, first order transitions to finite magnetic moments, not
visible as singularities of the paramagnetic susceptibility, may appear in the
very strong coupling regime. In d=3 second order transitions are found but only
for very strong couplings, where the non-locality of the electronic self-energy
acts in favour of the spontaneous magnetic moment. The influence of the
non-local part of the self-energy is particularly strong for lattices with
small coordination number. The non-bipartite fcc lattice exhibits saturated
ferromagnetism for all band occupations $1\le n\le 2$ while for less than half
filled bands ($0\le n\le 1$) the system remains in any case paramagnetic, and
that for d=3 as well as d=\infty. The Curie temperature runs through a maximum
at about $n=1.4$ and vanishes for $n\to 1$ and $n\to 2$.
