Span, Cospan, and Other Double Categories

Abstract: Given a double category D such that D_0 has pushouts, we characterize oplax/lax adjunctions between D and Cospan(D_0) such that the right adjoint is normal and restricts to the identity on D_0, where Cospan(D_0) denotes the double category on D_0 whose vertical morphisms are cospans. We show that such a pair exists if and only if D has companions, conjoints, and 1-cotabulators. The right adjoints are induced by the companions and conjoints, and the left adjoints by the 1-cotabulators. The notion of a 1-cotabulator is a common generalization of the symmetric algebra of a module and Artin-Wraith glueing of toposes, locales, and topological spaces. Along the way, we obtain a characterization of double categories with companions and conjoints as those for which the identity functor on D_0 extends to a normal lax functor from Cospan(D_0) to D.