cosimplicial

Cosimplicial refers to a mathematical concept that is dual to the notion of a simplicial object. In abstract mathematics, particularly in algebraic topology and category theory, a simplicial object is a sequence of objects indexed by the simplices, with face and degeneracy maps satisfying certain compatibility conditions. A cosimplicial object reverses the direction of these maps, leading to a structure where the maps go in the opposite direction.

More formally, a simplicial set is a functor from the simplex category to the category of sets, where the simplex category has a specific structure of generating objects (simplices) and a set of maps between them (face and degeneracy operators). A cosimplicial object, in contrast, is a functor from the opposite of the simplex category to the category of sets. This duality means that many constructions and theorems about simplicial objects have dual counterparts for cosimplicial objects.

Cosimplicial objects play a role in various areas of mathematics. For example, they are used in the study of cohomology theories, particularly in the context of Čech cohomology. They also appear in the development of derived functors and in the theory of resolutions. The dual nature of simplicial and cosimplicial structures allows for a deeper understanding of the underlying categorical principles.