- #1
AnalysisNewb
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Homework Statement
I came across a problem where f: (-π/2, π/2)→ℝ where f(x) = [itex]\sum\limits_{n=1}^\infty\frac{(sin(x))^n}{\sqrt(n)}[/itex]
The problem had three parts.
The first was to prove the series was convergent ∀ x ∈ (-π/2, π/2)
The second was to prove that the function f(x) was continuous over the same interval
The third was to prove that the function f(x) was differentiable over the interval
Homework Equations
The Attempt at a Solution
The first part of the problem was simple using the comparison test with the infinite sum of (sin(x))^n, and noting that the geometric series produced converges as long as |sin(x)|<1. Since |sin(x)| = 1 at |π/2|, it is convergent in the interval.
The second part of the problem, I ran into difficulty. I tried to show that f(x) is the limit of a series of continuous functions to prove the continuity of f(x), but then I remembered that for this to work, the series of continuous functions has to be uniformly convergent and not merely convergent.
So, I tried to prove the series was uniformly convergent using the Weierstass M test, but failed because I couldn't find a convergent sequence of M terms that were always greater than the absolute value of the terms of the series.
This same issue with convergent vs. uniformly convergent is what is plaguing me with the third part as well.
Any advice on how to work around this convergent vs. uniformly convergent issue would be greatly appreciated. If I can prove that the series is uniformly convergent, the second and third part are quite easy to prove using the Uniform Limit Theorem for part 2, and then the related theorems about differentiability of series that converge uniformly on an open interval for part 3.