Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Existence of sequence

  1. Nov 4, 2011 #1
    I asked this question before but I totally misunderstood what it was asking. Basically, I need to find that there exists a sequence {a_k} such that it converges to x for some x in R.

    Since the real numbers are equivalence classes of convergent Cauchy sequences the result seems fairly obvious, but I was thinking of instead of dealing with equivalence classes I could use the definition of the limit of a sequence and have x be the limit of {a_k}.

    I'm having some trouble starting up, though. Do I have to find a sequence that must converge to some arbitrary x to prove its existence?

    Thanks!
     
  2. jcsd
  3. Nov 4, 2011 #2

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Take the constant sequence [itex]a_k=x[/itex]. Surely, the sequence

    [tex]x,~x,~x~,...[/tex]

    converges to x...
     
  4. Nov 4, 2011 #3
    Sorry, forgot to mention that the sequence is in Q.
     
  5. Nov 4, 2011 #4

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Well, what can you go from?? Can you use that Q is dense in R?? Or must you use the definition of R?
     
  6. Nov 4, 2011 #5
    does the fact that Q is dense in R tell me that if I take any member of {s_n} it can exist in an interval of, say, (x-1/n,x+1/n) ?
     
  7. Nov 4, 2011 #6

    micromass

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Q being dense says that for each x in R, there exists a rational in (x-1/n,x+1/n). You can use this to construct a rational sequence.
     
  8. Nov 4, 2011 #7
    Can you just take successive truncations of x's decimal expansion? For example

    3, 3.1, 3.14, 3.141, 3.1415, ... is a sequence of rationals converging to pi.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Existence of sequence
  1. Existence of limits (Replies: 1)

Loading...