Suppose {an}n≥1 is a sequence of non-negative real numbers such that s=a1+...+an<∞.  For k ≥ 1, let Nk denote the cardinality of the set {n ∈ N : an ≥ 2^(-k)}. Show that k→∞ lim sup2^(-k)Nk=0

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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Suppose {an}n1 is a sequence of non-negative real numbers such

that s=a1+...+an<∞.  For k 1, let Nk denote the cardinality of the set

{n N : an 2^(-k)}.

Show that k→∞

lim sup2^(-k)Nk=0

 

 

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