There are the description of three conversations between Jean-Pierre Petit og 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).