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Real Analysis: Dyadic Series

  1. Nov 14, 2006 #1
    Can anyone give me any help on how to get started, or how to do this problem?
    Prove that if the terms of a sequence decrease monotonically (a_1)>= (a_2)>= ....
    and converge to 0 then the series [sum](a_k) converges iff the associated
    dyadic series (a_1)+2(a_2)+4(a_4)+8(a_8)+... = [sum](2^k)*(a_2^k) converges.

    I call this the block test b/c it groups the terms of the series in blocks
    of length 2^(k-1).
    thank you!
  2. jcsd
  3. Nov 14, 2006 #2


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    Can you show that this pair of inequalities is true:

    [tex]2 \times \sum_{i=1}^{2^k} a_n \geq \sum_{i=0}^{k} \left( \sum_{j=2^{i-1}}^{2^i} a_{2^{i-1}} \right) \geq \sum_{i=1}^{2^k} a_n[/tex]

    (The middle expression is the dyadic series.)
    Last edited: Nov 14, 2006
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