Dowker duality, profunctors, and spectral sequences

The intent of this paper is to explore Dowker duality from a combinatorial, topological, and categorical perspective. The paper presents three short, new proofs of Dowker duality using various poset fiber lemmas. We introduce modifications of joins and products of simplicial complexes called relational join and relational product complexes. These relational complexes can be constructed whenever there is a relation between simplicial complexes, which includes the context of Dowker duality and covers of simplicial complexes. In this more general setting, we show that the homologies of the simplicial complexes and the relational complexes fit together in a long exact sequence. Similar results are then established for profunctors, which are generalizations of relations to categories. The cograph and graph of profunctors play the role of the relational join and relational product complexes. For a profunctor that arise from adjoint functors between C and D, we show that C, D, the cograph, and the graph all have homotopy equivalent classifying spaces. Lastly, we show that given any profunctor from D to C, the homologies of C, D, the cograph, and the graph form a long exact sequence. We make use of a spectral sequence for Grothendieck fibrations of small categories.