Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

It's well known that filters and ideals are dual. I would like to see how to express this fact "Categorically". I would be very thankful if someone could help me with that.

share|cite|improve this question
That's not the sense in which filters and ideals are dual. Rather, a filter is a certain kind of subcategory of a poset (thought of as a category), and if you take the opposite of the poset, then filters become ideals. – Zhen Lin Oct 30 '12 at 22:21
There is such a thing as the category of filters, but as Zhen Lin points out, its dual has nothing to do with ideals. Its objects are pairs $(X,F)$ where $X$ is a set and $F$ is a filter on $X$. A morphism between objects $(X',F')$ and $(X,F)$ is a function $f:X'\rightarrow X$ with the property that $f^{-1}(A)\in F'$ whenever $A\in F$. – Shawn Henry Nov 1 '12 at 20:02
up vote 2 down vote accepted

The duality between filters and ideals can be seen as Zhen Lin describes, and thus answering your question negatively. A somewhat more positive answer (though only somewhat) is to notice that whatever category you concoct from filters, you can also concoct using ideals (in effect, using the duality between them). The resulting categories would then be isomorphic (not dual!).

Having said that, the answer to your question could turn out to be a positive one, I'm just not sure. The thing is that there are many ways to define the morphisms in a category whose objects are all pairs $(X,\mathcal F)$ where $X$ is a set and $\mathcal F$ is a filter on it. There seems to be quite a lot of flexibility on the choice of morphisms, so maybe one that suits your needs exists.

Just to clarify, there are two commonly considered categories of filters, described in Blass' article. These turn out to be very useful notions of categories (e.g., for one there is a natural notion of tensor product, the other is useful for constructive nonstandard analysis).

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.