The Zariski Cotangent Space at the Origin of the Pencil Diffeological Space

Abstract

The pencil diffeological space is a plane whose smooth structure at the origin is detected only along lines through it. We show that the Zariski cotangent space at the origin is entirely determined by the derivatives along these lines. Moreover, the derivatives along different lines form precisely a $σ$-continuous family, a weak form of continuity. We also compute the external and right tangent spaces at the origin from this description. This suggests that the local smooth structure at a singular point of a diffeological space can give rise to unexpected topological notions.