The geometric and numerical properties of duality in projective algebraic geometry (Q1110609)

After recalling standard definitions of the dual variety and the conormal scheme, well known classical numerical invariants (degrees of the polar classes) are introduced. The connections of these invariants and the Chern classes are discussed. Bringing together duality of hyperplane sections and projections permits simple proofs of results by \textit{R. Piene} [Ann. Sci. Éc. Norm. Supér., IV. Sér. 11, 247-276 (1978; Zbl 0401.14007)] and A. Hefez and \textit{S. L. Kleiman} [cf. Algebraic geometry, Proc. Conf., Vancouver 1984, CMS Conf. Proc. 6, 163-225 (1986; Zbl 0601.14046)].