8
$\begingroup$

Intuitively, my question is: how many times do we have to mod out by an closed equivalence relation with all classes compact in order to collapse Baire space $\omega^\omega$ to a singleton?

In more detail, given a space $\mathcal{X}$ let $\mathsf{CoLe}(\mathcal{X})$ be the least $\theta$ such that there is a sequence of spaces $(\mathcal{X}_\eta)_{\eta<\theta+1}$ where

  • $\mathcal{X}_0=\mathcal{X}$,

  • each $\mathcal{X}_{\eta+1}$ is the quotient of $\mathcal{X}_\eta$ by a closed equivalence relation each of whose classes is compact in the sense of $\mathcal{X}_\eta$,

  • for $\lambda<\theta$ a limit ordinal, the space $\mathcal{X}_\lambda$ is the colimit of the family of $\mathcal{X}_\eta$s with $\eta<\lambda$, and

  • $\mathcal{X}_\theta$ is a singleton.

For example, if we use $\mathbb{R}$ instead of $\omega^\omega$ then the corresponding ordinal is $\omega$: at stage $n$ we can collapse $[-n,n]$ to a point, and at stage $\omega$ this gives us the one-element space. Similarly, by collapsing just a pair of points at a time, we clearly have an upper bound of $\mathfrak{c}$, and it's not hard to show that $\omega_1$ is a lower bound for $\mathsf{CoLe}(\omega^\omega)$ (essentially this is an elaboration on the non-$\sigma$-compactness of $\omega^\omega$).

However, beyond that things aren't clear to me. In particular:

Is it consistent with $\mathsf{ZFC}$ that $\mathsf{CoLe}(\omega^\omega)<\mathfrak{c}$?

$\endgroup$

1 Answer 1

7
$\begingroup$

Baire space is the union of $\mathfrak d$ (the dominating number) compact subsets. So, using equivalence relations that collapse those sets one at a time (i.e., one equivalence class is the set to be collapsed and the other equivalence classes are singletons), we can collapse Baire space in at most $\mathfrak d$ steps. In particular, this can consistently be $\aleph_1$ steps while the continuum is large.

$\endgroup$
2
  • 1
    $\begingroup$ Oh that's clear! I feel silly for not having seen it. $\endgroup$ Nov 1 at 2:23
  • 1
    $\begingroup$ Hi Andreas, I sent you an email a couple of weeks ago, did it arrive safely? $\endgroup$
    – Asaf Karagila
    Nov 1 at 7:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

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