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looserlama
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Homework Statement
Suppose that the power series [itex]\sum[/itex]anxn for n=0 to n=∞ has a radius of convergence R[itex]\in[/itex](0,∞). Find the radii of convergence of the series [itex]\sum[/itex]anxn2 from n=0 to n=∞ and [itex]\sum[/itex]anx2n.
Homework Equations
Radius of convergence theorem:
R = 1/limsup|an|1/n is the radius of convergence of the series [itex]\sum[/itex]an(x-c)n
Root Test:
Let L = limsup|an|1/n, then
If L<1, the series [itex]\sum[/itex]an converges absolutely
If L>1, the series diverges
If L=1, then the test is inconclusive.
The Attempt at a Solution
So we know that limsup|an|1/n = [itex]\frac{1}{R}[/itex].
So for the second series ([itex]\sum[/itex]anx2n) I think this is easy:
limsup|an|1/n|x|2n/n = limsup|an|1/n|x|2 = |x|2/R < 1 So that the series converges
Therefore, |x|2 < R [itex]\Rightarrow[/itex] |x| < [itex]\sqrt{R}[/itex]
So R2 = [itex]\sqrt{R}[/itex] (Where R2 is the r.o.c. of the second series)So I tried doing this with the first series, but it doesn't work.
So then I showed that limsup|bn|1/n2 < 1 implies that [itex]\sum[/itex]bn from n=0 to n=∞ converges absolutely (basically a modification of the root test)
So then if I use this for the first series:
limsup|an|1/n2|x|n2/n2 = limsup|an|1/n2|x| < 1 So that it converges.
Therefore |x| < 1/limsup|an|1/n2 = R1
But I'm assuming they want it in terms of R, so this doesn't really work...
I feel like I'm close I'm just missing something?
Any help would be greatly appreciated.
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