ABSTRACT
Here we explain why there are fewer interesting possibilities than in the rectangular case and why a good choice of parameters in the triangular case is = 0, fa = 0 , . . . , (j>k = 0.
§2. Notation
Let £ (resp. T ) be a real affine space of dimension three (resp. four), and £ (resp. be its associated linear vector space so that £ (resp. £ ) is a hyperplane of T (resp. T ) . We denote by £ the real projective space associated to £.
