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Homework Help: Sequences and convergence in the standard topology

  1. Apr 3, 2012 #1
    Hello all.

    I have to present a proof to our Intro to Topology class and I just wanted to make sure I did it right (before I look like a fool up there).


    Let c be in ℝ such that c≠0. Prove that if {an} converges to a in the standard topology, denoted by τs, then {can} converges to ca in the standard topology on ℝ.


    Let c [itex]\in[/itex] ℝ such that c≠0. Suppose {an} converges to a in the standard topology, denoted by τs.

    Let V [itex]\in[/itex] τs with ca in V. Since V in τs, there exists an interval (p,q) with ca [itex]\in[/itex] (p,q) and (p,q) [itex]\subseteq[/itex] V. Thus, p < ca < q, which implies p/c < a < q/c.

    Note that p/c < (p/c + a)/2 < a < (q/c + a)/2 < q/c

    Thus, (p/c , q/c) [itex]\in[/itex] τs such that a [itex]\in[/itex] (p/c, q/c).

    Since, by our assumption, {an} converges to a in the standard topology, there exists m [itex]\in[/itex] N such that an [itex]\in[/itex] (p/c,q/c) for all n ≥ m. Hence, p/c < an < q/c. Hence, p < can< q for all n≥m. Since can [itex]\in[/itex] (p,q) and (p,q) [itex]\subseteq[/itex] V, can [itex]\in [/itex]V for all n ≥ m.

    Therefore, {can} converges to ca in the standard topology on ℝ.
  2. jcsd
  3. Apr 3, 2012 #2
    Looks pretty good. Two minor points.

    Beware that c might be negative.

    I don't really see the point of this line. Why is this necessary??

  4. Apr 3, 2012 #3
    Oh ok. So if I consider cases-- with c>0 and c<0-- then most of the work stays in tact. But in the case where c<0, when I divide by c, the inequalities will change, but this will later be undone when I multiply by c at the end. So it should still work. Correct?

    I mentioned that p/c < (p/c + a)/2 < a < (q/c + a)/2 < q/c because I needed to justify that (p/c, q/c) is in fact in the standard topology by showing that there exists an interval--specifically, the interval ( (p/c + a)/2, (q/c + a)/2 )-- such that a is in said interval and said interval is contained in (p/c,q/c).

    One thing is bothering me though...my prof always stressed that when prove that something "exists" when have to choose/set it. I get confused when using the definition of convergence since it implies that there exists an m\in N such that blah blah blah. By showing such an m exists, is that enough? Or do I have to find a specific m\in N where it works? Am I making any sense?
    Last edited: Apr 3, 2012
  5. Apr 3, 2012 #4
    That's ok.

    Ok, I see what you mean. But what is your definition of the standard topology?? Can't you just say that (p/c,q/c) is the interval contained in (p/c,q/c)?? Not that it matters much...

    The existence follows directly from the definition of convergence, no? I mean: if you know that an converges, then you automatically get the existence of an m such that blablabla.

    If you want to show convergence, then you need to find a specific m. But here you're already given convergence, so existence is no problem here.
  6. Apr 3, 2012 #5
    Oh duh! That makes sense and is a lot less complicated.

    Thanks for clearing that up. I appreciate your help =]
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