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Metric space question?
If a_n is a sequence greater than 0, and the sum of a_n converges, then the sum (n=1 to infinity) of sqrt(a_n)/n converges.
I had a similar problem to this one a few days ago. If we can show that both sqrt(a_n) and 1/n are both in L^2 (L^2 is the typical definition, defined as {s_n}(n=1 to infinity) : sum (n=1 to infinity) s_n^2 < infinity)
Since we know that a_n converges, and that it is obviously sqrt(a_n)^2, so sqrt(a_n) is in L^2., and (1/n)^2 = 1/n^2, and we know that that sums to pi^2/6, so 1/n is also in L^2. So, by Schwarz inequality, the product of them is absolutely convergent, and thus convergent as well.
1 Answer
- SuleimanLv 610 years agoFavorite Answer
a(n) is in ℓ¹ iff √(a(n)) is in ℓ².
And as you stated, 1/n is in ℓ².
So by the Schwarz inequality we have ||√(a(n))/n||₁ ≤ ||√(a(n))||₂||1/n||₂.
→ √(a(n))/n is in ℓ¹.