  The formal class of a germ of diffeomorphism $\phi$ is embeddable in a flow
if $\phi$ is formally conjugated to the exponential of a germ of vector field.
We prove that there are complex analytic unipotent germs of diffeomorphisms at
$({\mathbb C}^{n},0)$ ($n>1$) whose formal class is non-embeddable. The
examples are inside a family in which the non-embeddability is of geometrical
type. The proof relies on the properties of some linear functional operators
that we obtain through the study of polynomial families of diffeomorphisms via
potential theory.
