In this paper, we are
interested in the tangential Poisson cohomology (TP-cohomology) of regular Poisson
manifolds, a cohomology which was first defined by Lichnerowicz using contravariant
tensor fields. We show that for a regular Poisson manifold M, the TP-cohomology
coincides with the leafwise de Rham (or Cech) cohomology of the symplectic foliation
of M. Computing the spaces of such a cohomology leads actually to open and quite
nontrivial problems. To get a better understanding of these difficulties, we study
explicitly many examples coming from nilpotent and 3-dimensional (real) Lie
algebras. For the latter, we compare the TP-cohomology and the usual Poisson
cohomology (P-cohomology).