Sheaves are objects of a local nature: a global section is determined by how it looks locally. Hence, a sheaf cannot describe mathematical structures which contain global or nonlocal geometric information. To fill this gap, we introduce the theory of ``gleaves'', which are presheaves equipped with an additional ``gluing operation'' of compatible pairs of local sections. This generalizes the conditional product structures of Dawid and Studeny, which correspond to gleaves on distributive lattices. Our examples include the gleaf of metric spaces and the gleaf of joint probability distributions. A result of Johnstone shows that a category of gleaves can have a subobject classifier despite not being cartesian closed.
Gleaves over the simplex category $\Delta$, which we call compositories, can be interpreted as a new kind of higher category in which the composition of an m-morphism and an n-morphism along a common k-morphism face results in an (m+n-k)-morphism. The distinctive feature of this composition operation is that the original morphisms can be recovered from the composite morphism as initial and final faces. Examples of compositories include nerves of categories and compositories of higher spans.
Keywords: sheaf theory, uniqueness of gluing, nerve of a category, higher span, conditional product distribution, Lawvere metric space, relational database
2010 MSC: Primary 18F20, Secondary 18A99, 18F10
Theory and Applications of Categories, Vol. 31, 2016, No. 33, pp 928-988.