  The Mott-Hubbard metal-insulator transition is studied within a simplified
version of the Dynamical Mean-Field Theory (DMFT) in which the coupling between
the impurity level and the conduction band is approximated by a single pole at
the Fermi energy. In this approach, the DMFT equations are linearized, and the
value for the critical Coulomb repulsion U_{\rm c} can be calculated
analytically. For the symmetric single-band Hubbard model at zero temperature,
the critical value is found to be given by 6 times the square root of the
second moment of the free (U=0) density of states. This result is in good
agreement with the numerical value obtained from the Projective Selfconsistent
Method and recent Numerical Renormalization Group calculations for the Bethe
and the hypercubic lattice in infinite dimensions. The generalization to more
complicated lattices is discussed. The ``linearized DMFT'' yields plausible
results for the complete geometry dependence of the critical interaction.
