抄録
Reductive models ((\mathfrak{g}, \mathfrak{h}), H) for Cartan geometries are showed to fall into two classes, symmetric and non symmetric type, according to the existence or non existence of a mutation \mathfrak{g}'=\mathfrak{h}⊕\mathfrak{m} where the H-module m is an abelian subalgebra. Sasakian structures are showed to be Cartan geometries having a model of non symmetric type and other examples of models of this type are exhibited. Reductive models for which no Cartan space forms exist are constructed. The phenomenon of non-existence of Cartan space forms pertains to models of non symmetric type.
