0
$\begingroup$

I'm having difficulty with the meanings of these terms in several references. "Countable" may mean either a set with cardinality = N (i.e. countably infinite) , or with cardinality $\le$ N (i.e. countably infinite or finite). "Sequences" may mean specifically an infinite sequence (with cardinality = N), or may allow infinite or finite (with cardinality $\le$ N)

  1. .... Specifically, a space X is said to be first-countable if each point has a countable neighborhood basis (local base). That is, for each point x in X there exists a sequence $N_1, N_2, … $ of neighborhoods of x such that ....(Wiki)
  2. .... More explicitly, this means that a topological space T is second countable if there exists some countable collection $\mathcal{U} = \{U_i\}_{i=1}^\infty $ of open subsets of T... (Wiki)
  3. a Lindelöf space is a topological space in which every open cover has a countable subcover. The Lindelöf property is a weakening of the more commonly used notion of compactness, which requires the existence of a finite subcover.... (Wiki)
  4. In mathematics a topological space is called separable if it contains a countable, dense subset; that is, there exists a sequence $\{x_{n}\}_{n=1}^{\infty } $of elements of the space such that every nonempty open subset of the space contains at least one element of the sequence.... (Wiki - in this case it seems that the sequence is intended to be infinite).
  5. In mathematics, a countable set is a set with the same cardinality (number of elements) as some subset of the set of natural numbers. A countable set is either a finite set or a countably infinite set. ....Some authors use countable set to mean countably infinite alone (Wiki)

My confusion continues into, for example, the proof that a second countable metric space is Lindelöf, e.g. here: https://proofwiki.org/wiki/Sequentially_Compact_Metric_Space_is_Lindel%C3%B6f As far as I can see, the proof seems to establish that every cover has a "countable", possibly finite, subcover: should it also prove that the subcover is not finite ?

Improve this question
$\endgroup$
  • 2
    $\begingroup$ Every use of "countable" you've mentioned above is the loose (e.g., allowing finite) sense. This is, to the best of my knowledge, by far the most common meaning of "countable," except in contexts where it is clear that finite sets would provide trivial counterexamples. By contrast, "sequence" means "infinite sequence" almost always in topology (this one is a bit more ambiguous I think), although again it's usually clear from context what's meant. A convention I like a lot is to use "string" for finite sequence, and "sequence" for infinite sequences only; but I don't think this is universal. $\endgroup$ – Noah Schweber Dec 14 '15 at 16:08
  • $\begingroup$ @NoahSchweber Thanks, but it doesn't seem loose when linked to a infinite sequence as in (2) and (4). $\endgroup$ – Tom Collinge Dec 14 '15 at 16:10
  • $\begingroup$ In 2 and 4, the sequence is infinite but need not consist of distinct points/sets. $\endgroup$ – Noah Schweber Dec 14 '15 at 16:11
1
$\begingroup$

I don't follow what the problem is. In general, and here, countable means countably infinite or finite. Sequence (at least in this context) means "sequence indexed by the natural numbers". Maybe you need to note that the terms in a sequence need not be distinct; an "infinite sequence" may actually attain only finitely many values.

So for example say $X=\{0\}$, with the only possible topology. Then $X$ is first countable. The required sequence of neighborhoods of $0$ is $N_1,N_2,\dots$, where $N_j=X$ for all $j$.

Similarly a space with only finitely many points is separable. It's clear that such a space contains a countable dense subset. The sequence in the "that is" part of (4) would be any ("infinite") sequence that contains every element of $X$ infinitely often.

Improve this answer
$\endgroup$
  • $\begingroup$ To be fair, there are moments of ambiguity - for instance, if someone says "set of binary sequences," I'm not one hundred percent sure they mean $2^\omega$ instead of $2^{<\omega}$ (although they tend to, at least in logic) - but yes, all the mentioned examples use the loose (and correct IMO) sense of countable. $\endgroup$ – Noah Schweber Dec 14 '15 at 16:19
  • $\begingroup$ I think that some of the confusion is that the Wiki definition's references to sequences are unnecessary (I think !). The ProofWiki definitions consistently use countable as you have, and avoid sequences in these definitions:proofwiki.org/wiki/Definition:First-Countable_Space, proofwiki.org/wiki/Definition:Second-Countable_Space, proofwiki.org/wiki/Definition:Separable_Space $\endgroup$ – Tom Collinge Dec 14 '15 at 16:23
  • $\begingroup$ @NoahSchweber Fair enough. In logic, or maybe I should say set theory, I was confused the first time I saw "sequence" used to refer to any function defined on an ordinal... $\endgroup$ – David C. Ullrich Dec 14 '15 at 16:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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