## Functors

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# Functors

I will now briefly explain the concept of a functor.
Generally speaking a functor is a transformation from one category $\mathscr{C}$ to another category $\mathscr{D}$, such that the categorical structure of the domain $\mathscr{C}$ is preserved, i.e. gets mapped onto $\mathscr{D}$.
There are two types of functors:
1. Covariant Functor
2. Contravariant Functor
1. Definition 2.6   : A covariant functor from a category $\mathscr{C}$ to a category $\mathscr{D}$ is a map $F:\mathscr{C}\rightarrow\mathscr{D}$ that assigns, to each $\mathscr{C},$-object a $\mathscr{D}$-object F(a) and to each $\mathscr{C}$-arrow $f:a\rightarrow b$ a $\mathscr{D}$-arrow $F(f):F(a)\rightarrow F(b)$ such that the following are satisfied:
1. $F(1_a)=1_{F(a)}$
2. $F(f o g)=F(f) o F(g)$ for any $g:c\rightarrow a$

It is clear from the above that a covariant functor is a transformation that preserves both:
• the domain's and the codomain's identities
• the composites of functions, i.e. it preserves the direction of the arrows.
This can be easily seen with the aid of the following diagram:

$\displaystyle \xymatrix{ a\ar[rr]^f\ar[rrdd]_h&&b\ar[dd]^g\\ &&\\ &&c\\ } \xymatrix{\ar@{=>}[rr]^F&&} \xymatrix{ F(a)\ar[rr]^{F(f)}\ar[rrdd]_{F(h)}&&F(b)\ar[dd]^{F(g)}\\ &&\\ &&F(c)\\ }$

2. Definition 2.7:

a contravariant functor from a category $\mathscr{C}$ to a category $\mathscr{D}$ is a map $X:\mathscr{C}\rightarrow\mathscr{D}$ that assigns to each $\mathscr{C}$-object a, a $\mathscr{D}$-object X(a) and to each $\mathscr{C}$-arrow $f:a\rightarrow b$ a $\mathscr{D}$-arrow $X(f):X(b)\rightarrow X(a),$ such that the following are satisfied

1. $X(1_a)=1_{X(a)}$
2. $X(f o g)=X(g) o X(f)$ for any $g:c\rightarrow a$

A diagrammatic representation of a contravariant functor is the following:

$\displaystyle \xymatrix{ a\ar@{>}[rr]^f\ar[rrdd]_h&&b\ar[dd]^g\\ &&\\ &&c\\ } \xymatrix{\ar@{=>}[rr]^X&&} \xymatrix{ F(a)&&X(b)\ar[ll]^{X(f)}\\ &&\\ &&X(c)\ar[lluu]^{X(h)}\ar[uu]^{X(g)}\\ }$

As we can see from the above diagram a contravariant functor, in mapping arrows from one category to the next, it reverses the directions of the arrows by mapping domains to codomains and vice versa.

Next: Bibliography Up: Appendix Previous: Terminal Object
Cecilia Flori 2006-11-20