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Homework Help: Convergent Series Proof

  1. Oct 31, 2008 #1
    1. The problem statement, all variables and given/known data

    If the sequence of partial sums of |a_n| is convergent and b_n is bounded, prove that the sequence of partial sums of the product (a_n)(b_n) is also convergent.

    2. Relevant equations

    Cauchy sequences and bounded sequences



    3. The attempt at a solution

    I wrote the following

    for n,m > N_1 and e > 0

    |a_n - a_m | < e

    which proves that a_n is a cauchy sequence, for every convergent sequence is a cauchy sequence.

    For b_n, we assume e = 1, and for n, m > N_2

    we have

    |b_n - b_m | < 1

    then for N = max{N_1, N_2}

    |a_n * b_n - a_m * b_m | < e + 1
     
  2. jcsd
  3. Oct 31, 2008 #2

    HallsofIvy

    User Avatar
    Science Advisor

    Well, no. "|a_n- a_m|< e" doesn't prove that a_n is a Cauchy sequence. The fact that every convergent sequence is a Cauchy sequence proves that |a_n- a_m|< e.
    Also, the wording "for n,m> N_1 and e> 0" is incorrect. That implies that you can find m,m> N_1 such |a_n- a_m|< e for all e> 0 which is not true. Given e> 0 first, you can find N_1 such that that is true.

    This is certainly NOT true. Your only condition on {b_n} is that it is bounded, not that it is convergent. For example, {b_n}= {(-1)^n} is bounded and this is not true

    What does "bounded" mean?
     
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