Modal languages for topology: expressivity and definability
  In this paper we study the expressive power and definability for (extended) modal languages interpreted on topological spaces. We provide topological analogues of the van Benthem characterization theorem and the Goldblatt-Thomason definability theorem in terms of the well established first-order topological language $L_t$. 
Balder ten Cate (University of Amsterdam), David Gabelaia (Razmadze   Mathematical Institute), Dmitry Sustretov (INRIA Lorra...