Categorified presheaves and sieves
  Let $\mathcal C$ be a category of a set of (small) categories. This paper concerns with the ${\mathbf {Cat}}$-valued presheaves and sieves over category $\mathcal C.$ Since ${\mathbf {Cat}}$ is not a concrete category, existing definition of presheaves can not deal with the situation. This paper proposes a new framework for the purpose. The main result is a version of Yoneda embedding for ${\mathbf {Cat}}$-valued presheaves, and construction of the ${\mathbf {Cat}}$-valued sieves over the category ${\mathcal O}(\mathbf B)$ of subcategories of a given topological category $\mathbf B.$ 
Saikat Chatterjee
math.CT