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Friday, May 16, 2014

Rearrangement of a Different Convergence (Example 3.53)

Walter Rudin Principles of Mathematical Analysis, chapter 3, example 53:

MathJax TeX Test Page Consider the convergent series 11/2+1/31/4+... and one of its rearrangements 1+1/31/2+1/5+1/71/4+1/9+1/111/6+... As we have seen, the rearrangement sn if it converges does so at a point larger than the point of convergence for the original series. Prove sn converges.

Proof: For any sequence an, let f :  be such that af(n) and anaf(n) are convergent sequences. Then we see (lim anaf(n))+(lim af(n))=lim an so that an converges. As it applies to this problem, we will show the subsequence s3n converges, and since lim snsf(n)=0 where f(n) rounds n to the next highest multiple of 3, sn converges (technically sf(n) has different terms from s3n, but lim sf(n)=lim s3n since the former merely has finite copies of each of the latter's terms).

It suffices to compare the sum's terms an to 4n2, which forms a convergent series: |an|=an=14n3+14n112n=8n32n(4n3)(4n1)4(4n3)(4n1)4n2  

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