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Another Sequence proof too trivial to figure out

  1. Dec 6, 2009 #1
    1. The problem statement, all variables and given/known data
    Let Sn be a convergent sequence and the limit of Sn > a. Prove that there exists a number N such that when n > N implies Sn > a


    2. Relevant equations

    sequence definition of limit

    3. The attempt at a solution

    This one does not seem to have a point to me. By definition when n > N the sequence is epsilon close to a and they tell us the limit is greater than a so once n>N we are close to the lim of Sn so we must be above a. Am I missing something?
     
  2. jcsd
  3. Dec 6, 2009 #2

    Dick

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    Say lim Sn=b. And b>a. Just use the distance from b to a to make a particular choice of an epsilon.
     
  4. Dec 6, 2009 #3
    I would let epsilon be less than distance between a and b where b = lim Sn. With that selected as my epsilon can I use the fact that Sn being convergen means it can be slower than d(a,b) therefore that N exists?
     
  5. Dec 6, 2009 #4

    Dick

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    Well, yes. Pick e.g. epsilon=|b-a|/2. Now write out the proof.
     
  6. Dec 7, 2009 #5
    I think I got it now, my one question is why can't i just say epsilon less than d(a,b). That way when I say Sn is convergent b - epsilon is still bigger than a. You have d(a,b)/2. I thnk for the same reason but i might be overlooking something. Thanks for your help.
     
  7. Dec 7, 2009 #6

    HallsofIvy

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    No reason why you couldn't. If b= lim Sn> a, and [itex]\epsilon= d(a,b)= b-a[/itex] then there exist N such that if n>N then |Sn-a|< \epsilon= b-a. Then -(b-a)= a- b< b- Sn< b-a so, subtracting b from each part a- 2b< -Sn< -a. Multiplying each part by -1, 2b-a> Sn> a.
     
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