  This is an introduction to the hyperderminant, according to Gelfand, Kapranov
and Zelevinsky. The "triangle inequality", characterizing the Segre varieties
such that their dual variety is a hypersurface, is proved in a geometric way
(Theorems 3.3 and 6.2). Significant emphasis is given to boundary format case,
to pencil of quadrics and to the rank of multidimensional matrices. In the last
section we propose three open problems.
