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The theory of lattices in the sense of order theory. For the number-theoretic notion, use the tag "lattices" instead.
2
votes
How big is the lattice of all functions?
Here are a few tricks to play with.
For every $A\subseteq\omega$ define $f_A$ by $f_A(0)=0$, $f_A(n+1)=f_A(n)+1$ if $n\in A$, and $f_A(n+1)=f_A(n)$ if $n\notin A$.
If $A\subset^*B$ (so $B\setminus A$ …
3
votes
Accepted
Relationship of ${\cal P}(\omega)/fin$ and ${\cal L}$
Look at your and my second answer to the original question.
Take your map from $\mathcal{P}(\omega)$ into $\mathcal{L}$ (or rather the set of functions before identifying almost equal elements).
That …
5
votes
Accepted
A problem of non-emptiness of intersections of certain chains of regular open sets
Here is a provisional negative answer.
If $\mathcal{C}$ is a c-minimal chain then $N=\bigcap\mathcal{C}$ is closed and nowhere dense (if nonempty): if the interior is nonempty take a nonempty regular …
6
votes
Accepted
Can the Boolean Algebra of regular open sets be isomorphic to ${\cal P}(\omega)/(\text{fin})$?
No, $\mathrm{RO}(X)$ is complete; $\mathcal{P}(\omega)/\mathit{fin}$ is not (no strictly increasing sequence has a supremum).
7
votes
Accepted
Can any poset of cardinality $\leq 2^{\aleph_0}$ be embedded in ${\cal P}(\omega)/(\text{fin...
Here is an attempt at a 'definitive summary'.
To begin with positive results: $\mathsf{CH}$ implies a “yes” answer to
this question. The fastest way to see this is to first embed a given partial
order …