Wednesday, 10 October 2018

real analysis - Show that if bkuparrowinfty and Sigmaik=1nftyakbk converges, then bmSigmaik=mnftyak0 as minfty.

Suppose that Σk=1ak converges. Prove that if bk and Σk=1akbk converges, then



bmΣk=mak0 as m.



Attemtp: Suppose Σk=1ak converges, and Σk=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=Σj=kajbj. Then Σmk=nak=Σakbkbk=Σmk=nckck+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 nm1 set An,m=Σnk=mak
Σmk=nakbk=An,mbnΣn1k=mAk,m(bk+1bk)

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