  We consider a family of Caffarelli-Kohn-Nirenberg interpolation inequalities
and weighted logarithmic Hardy inequalities which have been obtained recently
as a limit case of the first ones. We discuss the ranges of the parameters for
which the optimal constants are achieved by extremal functions. The comparison
of these optimal constants with the optimal constants of Gagliardo-Nirenberg
interpolation inequalities and Gross' logarithmic Sobolev inequality, both
without weights, gives a general criterion for such an existence result in some
particular cases.
