Sunday, 9 September 2007

ct.category theory - Does the category of pro-sets have a generator?

Consider for simplicity the category of pointed sets $Set_*$. Let's call an object $G$ of a pointed category $C$ a generator if for every nonzero object $X in C$ there is a nonzero map $G to X$. In the category of pointed sets, for example, the two-point set is a generator (and a cogenerator as well).



Does the category $Pro-Set_*$ of cofiltered diagrams of pointed sets have a generator as well? If $X$ is a pro-pointed set with nontrivial limit then there is a nontrivial map $G to X$, where $G$ is a generator of pointed sets, considered as a one-object diagram. However, there are many pro-sets with trivial limit. For example, the pro-set $Xcolon mathbb N to Set$ given by $X(i) = mathbb N$ and $X(i+1 to i)(n) = n+1$ is a pro-set with empty limit, but which is nontrivial -- add a point in every degree to get an example in pointed sets.



Thus, my question is: Does the category of pro-pointed sets have a generator?



And, if the answer was yes, does more generally $Pro-C$ have a generator whenever $C$ has one?

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