Radius of convergence of $sumlimits_{n=1}^infty((frac{1}{4})^n+(frac{1}{3})^n)x^n$
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according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:
$a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$
$implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$
$left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$
But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?
Hints to a better proof are greatly appreciated! :)
real-analysis sequences-and-series power-series
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add a comment |
$begingroup$
according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:
$a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$
$implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$
$left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$
But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?
Hints to a better proof are greatly appreciated! :)
real-analysis sequences-and-series power-series
$endgroup$
add a comment |
$begingroup$
according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:
$a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$
$implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$
$left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$
But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?
Hints to a better proof are greatly appreciated! :)
real-analysis sequences-and-series power-series
$endgroup$
according to wolfram alpha the radius is 3. I'm struggeling with the proof and would be very glad if someone could take a short look. Here my approach using the root test:
$a_n:=sqrt[n]{(frac{1}{4})^n+(frac{1}{3})^n}<sqrt[n]{(frac{1}{3})^n+(frac{1}{3})^n}=sqrt[n]{2(frac{1}{3})^n}=sqrt[n]2(frac{1}{3})=:b_n$
$implies limsuplimits_{nrightarrowinfty}a_n<limsuplimits_{nrightarrowinfty}b_n=frac{1}{3}$
$left(implies forall x in mathbb{R}: |x|<3 implies exists alpha in mathbb{R}: limlimits_{nrightarrowinfty}sum_{n=1}^infty b_nx^n=alpha right)$
But that does not necessarily mean that $limlimits_{nrightarrowinfty}sum_{n=1}^infty a_nx^n$ has the same radius of convergence, does it? The radius could still be smaller, right?
Hints to a better proof are greatly appreciated! :)
real-analysis sequences-and-series power-series
real-analysis sequences-and-series power-series
edited Dec 4 '18 at 22:18
Did
246k23221457
246k23221457
asked Dec 4 '18 at 22:00
DDevelopsDDevelops
533
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3 Answers
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Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
$$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$
Hence, $R=3$.
$endgroup$
add a comment |
$begingroup$
You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit
$$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$
exists then $rho$ is the radius of convergence of the power series. In this case we have that
$$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$
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This is the ratio test, not the root test (and yes, it works smoothly in the present case).
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– Did
Dec 4 '18 at 22:16
add a comment |
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We have that
$$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$
and therefore the radius of convergence is $3$.
Moreover note that
for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$
for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$
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Why the downvote, something wrong?
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– gimusi
Dec 4 '18 at 22:21
add a comment |
Your Answer
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3 Answers
3
active
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3 Answers
3
active
oldest
votes
active
oldest
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active
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votes
$begingroup$
Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
$$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$
Hence, $R=3$.
$endgroup$
add a comment |
$begingroup$
Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
$$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$
Hence, $R=3$.
$endgroup$
add a comment |
$begingroup$
Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
$$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$
Hence, $R=3$.
$endgroup$
Better if you compute $lim_{ntoinfty}frac{a_{n+1}}{a_n}$. In your case,
$$frac{1/4^{n+1}+1/3^{n+1}}{1/4^n+1/3^n}cdot frac{3^{n+1}}{3^{n+1}} =frac{(3/4)^{n+1}+1}{3cdot (3/4)^n+3}overset{ntoinfty}{longrightarrow} frac{1}{3}. $$
Hence, $R=3$.
answered Dec 4 '18 at 22:11
Tito EliatronTito Eliatron
1,448622
1,448622
add a comment |
add a comment |
$begingroup$
You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit
$$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$
exists then $rho$ is the radius of convergence of the power series. In this case we have that
$$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$
$endgroup$
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
add a comment |
$begingroup$
You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit
$$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$
exists then $rho$ is the radius of convergence of the power series. In this case we have that
$$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$
$endgroup$
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
add a comment |
$begingroup$
You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit
$$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$
exists then $rho$ is the radius of convergence of the power series. In this case we have that
$$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$
$endgroup$
You can use also the ratio test to get the radius of convergence, I mean, if $a_nneq 0$ for all $nge N$ for enough large $NinBbb N$ and the limit
$$rho=lim_{ntoinfty}frac{|a_n|}{|a_{n+1}|}$$
exists then $rho$ is the radius of convergence of the power series. In this case we have that
$$frac{|a_n|}{|a_{n+1}|}=frac{frac{4^n+3^n}{4^n 3^n}}{frac{4^{n+1}+3^{n+1}}{4^{n+1}3^{n+1}}}=12frac{4^n+3^n}{4^{n+1}+3^{n+1}}=12frac{1+(3/4)^n}{4+3(3/4)^n}tofrac{12}{4}=3$$
edited Dec 4 '18 at 22:16
answered Dec 4 '18 at 22:16
MasacrosoMasacroso
13k41746
13k41746
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
add a comment |
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
$begingroup$
This is the ratio test, not the root test (and yes, it works smoothly in the present case).
$endgroup$
– Did
Dec 4 '18 at 22:16
add a comment |
$begingroup$
We have that
$$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$
and therefore the radius of convergence is $3$.
Moreover note that
for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$
for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$
$endgroup$
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
add a comment |
$begingroup$
We have that
$$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$
and therefore the radius of convergence is $3$.
Moreover note that
for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$
for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$
$endgroup$
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
add a comment |
$begingroup$
We have that
$$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$
and therefore the radius of convergence is $3$.
Moreover note that
for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$
for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$
$endgroup$
We have that
$$|x|cdotsqrt[n]{left(frac{1}{4}right)^n+left(frac{1}{3}right)^n}=|x|cdotfrac13sqrt[n]{left(frac{3}{4}right)^n+1} to|x|cdotfrac13cdot 1 = |x|cdotfrac13<1$$
and therefore the radius of convergence is $3$.
Moreover note that
for $x=3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(frac{3}{4}right)^n+1$
for $x=-3 implies left(left(frac{1}{4}right)^n+left(frac{1}{3}right)^nright)x^n=left(-frac{3}{4}right)^n+(-1)^n$
edited Dec 4 '18 at 22:18
answered Dec 4 '18 at 22:11
gimusigimusi
1
1
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
add a comment |
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
$begingroup$
Why the downvote, something wrong?
$endgroup$
– gimusi
Dec 4 '18 at 22:21
add a comment |
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