Category of elements

In category theory, if C is a category and $F:C\to {\mathbf {Set}}$ is a set-valued functor, the category of elements of F ${\mathop {{\rm {el}}}}(F)$ (also denoted by ∫^CF) is the category defined as follows:

Objects are pairs $(A,a)$ where $A\in {\mathop {{\rm {Ob}}}}(C)$ and $a\in FA$ .
An arrow $(A,a)\to (B,b)$ is an arrow $f:A\to B$ in C such that $(Ff)a=b$ .

A more concise way to state this is that the category of elements of F is the comma category $\ast \downarrow F$ , where $\ast$ is a one-point set. The category of elements of F comes with a natural projection ${\mathop {{\rm {el}}}}(F)\to C$ that sends an object (A,a) to A, and an arrow $(A,a)\to (B,b)$ to its underlying arrow in C.

The category of elements of a presheaf

Somewhat confusingly in some texts (e.g. Mac Lane, Moerdijk), the category of elements for a presheaf is defined differently. If $P\in {\hat C}:={\mathbf {Set}}^{{C^{{op}}}}$ is a presheaf, the category of elements of P (again denoted by ${\mathop {{\rm {el}}}}(P)$ , or, to make the distinction to the above definition clear, ∫_C P) is the category defined as follows:

Objects are pairs $(A,a)$ where $A\in {\mathop {{\rm {Ob}}}}(C)$ and $a\in P(A)$ .
An arrow $(A,a)\to (B,b)$ is an arrow $f:A\to B$ in C such that $(Pf)b=a$ .

As one sees, the direction of the arrows is reversed. One can, once again, state this definition in a more concise manner: the category just defined is nothing but $(\ast \downarrow P)^{{{\rm {op}}}}$ . Consequentially, in the spirit of adding a "co" in front of the name for a construction to denote its opposite, one should rather call this category the category of coelements of P.

For C small, this construction can be extended into a functor ∫_C from ${\hat C}$ to ${\mathbf {Cat}}$ , the category of small categories. In fact, using the Yoneda lemma one can show that ∫_CP $\cong {\mathop {{\textbf {y}}}}\downarrow P$ , where ${\mathop {{\textbf {y}}}}:C\to {\hat {C}}$ is the Yoneda embedding. This isomorphism is natural in P and thus the functor ∫_C is naturally isomorphic to ${\mathop {{\textbf {y}}}}\downarrow -:{\hat C}\to {\textbf {Cat}}$ .

References

Mac Lane, Saunders (1998). Categories for the Working Mathematician. Graduate Texts in Mathematics 5 (2nd ed.). Springer-Verlag. ISBN 0-387-98403-8.
Mac Lane, Saunders; Moerdijk, Ieke (1992). Sheaves in Geometry and Logic. Universitext (corrected ed.). Springer-Verlag. ISBN 0-387-97710-4.

External links

Category of elements in nLab

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Category of elements

The category of elements of a presheaf

See also

References

External links