Set. 8. For the identity functor Ie of any category, the natural transformations CI. : Ie""'"'> Ie form a commutative monoid. Find this monoid in the cases C = Grp, Ab, and Set. 0 6. Comma Categories There is another general construction of a category whose objects are certain arrows, as in the following several special cases. If b is an object of the category C, the category of objects under b is the category (b !
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