There are the description of three conversations between Jean-Pierre Petit E Jacques Lacan. They took place in 1979 not so far from his death.
As you will see the themes of these conversations analyse the topology of the cross-cap and the Boy surface. There Lacan is focused on the points in these structures that actually do not exist but are a consequence of the immersions of a projective plane. A projective plane cannot be embedded in our 3D space (R³), it can only be embedded in a 4D space (R⁴). The interesting point is that Jean-Pierre Petit insists on the fact that these points do not exist in the reality of the structure, but Lacan still insists on these geometrical artifacts. This led to a lot of misinterpretations of this structure (Point hors-ligne, Ligne sans-points).