  We consider a Kolmogorov-Fokker-Planck operator of the kind studied by
Lanconelli-Polidoro in [Rend. Sem. Mat. Univ. Politec. Torino 52 (1994)], where
the leading coefficients $a_{ij}$, instead of being constant, are bounded
measurable functions of t. We construct an explicit fundamental solution for
this operator, study its property, show a comparison result between this
function and the fundamental solution of some model operators with constant
$a_{ij}$, and show the unique solvability of the Cauchy problem under various
assumptions on the initial datum.
