Let is a set. A filter (on ) is a non-empty set of subsets of such that . Note that unlike some other authors I do not require .

I will call *the set of filter objects* the set of filters ordered reverse to set theoretic inclusion of filters, with principal filters equated to the corresponding sets. See here for the formal definition of filter objects. I will denote the filter corresponding to a filter object . I will denote the set of filter objects (on ) as .

I will denote the set of atomic lattice elements under a given lattice element . If is a filter object, then is essentially the set of ultrafilters over .

**Problem**Which of the following expressions are pairwise equal for all for each set ? (If some are not equal, provide counter-examples.)

- \item ;

\item ;

\item ;

\item .

I have proved all equalities true.

## Bibliography

*Victor Porton. Open problem: Pseudodifference of filters

* indicates original appearance(s) of problem.