Characterization of the tangent space of monotone transport plans in   ${\R}\times{\R}$ with prescribed first projection
  We provide a complete description of the tangent space of the cone of monotone plans in $\R\times \R$ with prescribed first projection. We show that elements of this tangent space are essentially made of two simple building-block types of measures. 
Marc Sedjro and Michael Westdickenberg
math.MG math.AP