-Whittaker vectors corresponding to a principal nilpotent orbit of a real reductive linear Lie group, and wave front sets
To each complex number is associated a representation of the conformal group on (spherical principal series). For three values , we construct a trilinear form on , which is invariant by . The trilinear form, first defined for in an open set of is extended meromorphically, with simple poles located in an explicit family of hyperplanes. For generic values of the parameters, we prove uniqueness of trilinear invariant forms.