Proof: For any sequence an, let f : ℕ→ℕ be such that af(n) and an−af(n) are convergent sequences. Then we see (lim an−af(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 sn−sf(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=14n−3+14n−1−12n=8n−32n(4n−3)(4n−1)≤4(4n−3)(4n−1)≤4n2 ◻
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