Suppose that Σ∞k=1ak converges. Prove that if bk↑∞ and Σ∞k=1akbk converges, then
bmΣ∞k=mak→0 as m→∞.
Attemtp: Suppose Σ∞k=1ak converges, and Σ∞k=1akbk also.Then we know by Abel's Formula that the sequences converge only iff its partial sums converge. If we let Σak=Σakbkbk, then because bk is increasing we have 1bk is decreasing to zero. So we almost have a telescoping series.
Then let ck=Σ∞j=kajbj. Then Σmk=nak=Σakbkbk=Σmk=nck−ck+1bk
I don't know how to continue. I am having trouble applying Abel's Formula and the subscripts are confusing. Can someone please help me? I am suppose to use Abel's Formula. Thank you very much.
Abel's Formula: Let ak,bk be real sequences, and for each pair of integers n≥m≥1 set An,m=Σnk=mak
Σmk=nakbk=An,mbn−Σn−1k=mAk,m(bk+1−bk)
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