Saturday, 26 January 2008

ct.category theory - Is a functor which has a left adjoint which is also its right adjoint an equivalence ?

There are lots of examples. Here's what I think is in some sense the minimal one.



Let C be the terminal category mathbf1 (one object, and only the identity arrow). Then for any category D, a left adjoint to the unique functor G:Dtomathbf1 is an initial object of D, and a right adjoint is a terminal object. So, we're looking for a category D that has a zero object (one that is both initial and terminal), but is not equivalent to the terminal category.



There are plenty of such categories D, e.g. mathbfVect. But I guess the minimal one is the category D generated by a split epimorphism. In other words, it consists of two objects, 0 and d, and non-identity arrows
p:dto0,i:0tod,ip:dtod,


satisfying pi=10. Then 0 is a zero object but D is not equivalent to the terminal category.

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