Definition
Let F : J → C be a diagram in C. Formally, a diagram is nothing more than a functor from J to C. The change in terminology reflects the fact that we think of F as indexing a family of objects and morphisms in C. The category J is thought of as an "index category". One should consider this in analogy with the concept of an indexed family of objects in set theory. The primary difference is that here we have morphisms as well.
Let N be an object of C. A cone from N to F is a family of morphisms
for each object X of J such that for every morphism f : X → Y in J the following diagram commutes:
The (usually infinite) collection of all these triangles can be (partially) depicted in the shape of a cone with the apex N. The cone ψ is sometimes said to have vertex N and base F.
One can also define the dual notion of a cone from F to N (also called a co-cone) by reversing all the arrows above. Explicitly, a cone from F to N is a family of morphisms
for each object X of J such that for every morphism f : X → Y in J the following diagram commutes:
Read more about this topic: Cone (category Theory)
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