  In the recent progress [BE1], [M], [Z1] and [Z2], the well-known Jacobian
conjecture ([BCW], [E]) has been reduced to a problem on HN (Hessian nilpotent)
polynomials (the polynomials whose Hessian matrix are nilpotent) and their
(deformed) inversion pairs. In this paper, we prove several results on HN
polynomials, their (deformed) inversion pairs as well as the associated
symmetric polynomial or formal maps. We also propose some open problems for
further study of these objects.
