Show with the direct Comparison Test that $sqrt[n]{|a_n|}leqtheta$ converges absolutely
$begingroup$
Let $sum_{n=1}^{n=infty}{a_n}$ be an infinite series of real numbers. There is a $theta$ with $0<theta<1$ and a $n_0 > 0$, so $$sqrt[n]{|a_n|}leqtheta$$ for all $n ge n_0$. Show that the series converges absolutely.
So I need verification here. Quite funny, because this is the first task out of many and I am certain that the other ones are correct, but this one I am not sure of.
My proof feels wrong, it feels like as if it is way too short.
My proof here:
Since $lvert a_{n} rvert leq theta$ and $theta in (0,1)$, the series $sum_{n=1}^{n=infty}{a_n}$ converges. And also since $sqrt[n]{|a_n|}≤a_n$, the series converges absolutely.
real-analysis proof-verification convergence proof-writing absolute-convergence
$endgroup$
add a comment |
$begingroup$
Let $sum_{n=1}^{n=infty}{a_n}$ be an infinite series of real numbers. There is a $theta$ with $0<theta<1$ and a $n_0 > 0$, so $$sqrt[n]{|a_n|}leqtheta$$ for all $n ge n_0$. Show that the series converges absolutely.
So I need verification here. Quite funny, because this is the first task out of many and I am certain that the other ones are correct, but this one I am not sure of.
My proof feels wrong, it feels like as if it is way too short.
My proof here:
Since $lvert a_{n} rvert leq theta$ and $theta in (0,1)$, the series $sum_{n=1}^{n=infty}{a_n}$ converges. And also since $sqrt[n]{|a_n|}≤a_n$, the series converges absolutely.
real-analysis proof-verification convergence proof-writing absolute-convergence
$endgroup$
1
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
1
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32
add a comment |
$begingroup$
Let $sum_{n=1}^{n=infty}{a_n}$ be an infinite series of real numbers. There is a $theta$ with $0<theta<1$ and a $n_0 > 0$, so $$sqrt[n]{|a_n|}leqtheta$$ for all $n ge n_0$. Show that the series converges absolutely.
So I need verification here. Quite funny, because this is the first task out of many and I am certain that the other ones are correct, but this one I am not sure of.
My proof feels wrong, it feels like as if it is way too short.
My proof here:
Since $lvert a_{n} rvert leq theta$ and $theta in (0,1)$, the series $sum_{n=1}^{n=infty}{a_n}$ converges. And also since $sqrt[n]{|a_n|}≤a_n$, the series converges absolutely.
real-analysis proof-verification convergence proof-writing absolute-convergence
$endgroup$
Let $sum_{n=1}^{n=infty}{a_n}$ be an infinite series of real numbers. There is a $theta$ with $0<theta<1$ and a $n_0 > 0$, so $$sqrt[n]{|a_n|}leqtheta$$ for all $n ge n_0$. Show that the series converges absolutely.
So I need verification here. Quite funny, because this is the first task out of many and I am certain that the other ones are correct, but this one I am not sure of.
My proof feels wrong, it feels like as if it is way too short.
My proof here:
Since $lvert a_{n} rvert leq theta$ and $theta in (0,1)$, the series $sum_{n=1}^{n=infty}{a_n}$ converges. And also since $sqrt[n]{|a_n|}≤a_n$, the series converges absolutely.
real-analysis proof-verification convergence proof-writing absolute-convergence
real-analysis proof-verification convergence proof-writing absolute-convergence
edited Dec 1 '18 at 21:28
Bernard
119k639112
119k639112
asked Dec 1 '18 at 21:24
SacredScoutSacredScout
477
477
1
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
1
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32
add a comment |
1
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
1
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32
1
1
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
1
1
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
We have $$sqrt[n]{|a_n|}le thetato 0le |a_n|le theta^n$$therefore $$0le sum_{n=1}^{infty}|a_n|le sum_{n=1}^{infty}theta ^n={thetaover 1-theta}$$which completes the proof.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
add a comment |
$begingroup$
We are told that $|a_n|^{frac1n}leq theta$ whenever $n geq n_0$. So if $n geq n_0$, it then follows that we have $0leq |a_n|leqtheta^n$. Since $theta in (0,1)$, it also follows that $sum_{n=1}^infty theta^n$ is a convergent geometric series. Now the comparison test seems relevant...
Always check to make sure you are using the hypotheses of the problem, otherwise things tend to go agley.
$endgroup$
add a comment |
$begingroup$
$sqrt[n]{mid a_nmid}ltthetaimplies mid a_nmidletheta ^nimplies sum_{n=0}^infty mid a_nmidlesum_{n=0}^inftytheta^n=frac 1{1-theta}ltinfty $, since $theta lt1$.
Thus the series converges absolutely by comparison with a geometric series.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
We have $$sqrt[n]{|a_n|}le thetato 0le |a_n|le theta^n$$therefore $$0le sum_{n=1}^{infty}|a_n|le sum_{n=1}^{infty}theta ^n={thetaover 1-theta}$$which completes the proof.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
add a comment |
$begingroup$
We have $$sqrt[n]{|a_n|}le thetato 0le |a_n|le theta^n$$therefore $$0le sum_{n=1}^{infty}|a_n|le sum_{n=1}^{infty}theta ^n={thetaover 1-theta}$$which completes the proof.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
add a comment |
$begingroup$
We have $$sqrt[n]{|a_n|}le thetato 0le |a_n|le theta^n$$therefore $$0le sum_{n=1}^{infty}|a_n|le sum_{n=1}^{infty}theta ^n={thetaover 1-theta}$$which completes the proof.
$endgroup$
We have $$sqrt[n]{|a_n|}le thetato 0le |a_n|le theta^n$$therefore $$0le sum_{n=1}^{infty}|a_n|le sum_{n=1}^{infty}theta ^n={thetaover 1-theta}$$which completes the proof.
edited Dec 1 '18 at 22:16
Chris Custer
11.2k3824
11.2k3824
answered Dec 1 '18 at 21:34
Mostafa AyazMostafa Ayaz
15.2k3939
15.2k3939
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
add a comment |
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:04
1
1
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
$begingroup$
I simply used Geometric Sum (en.wikipedia.org/wiki/Geometric_series)
$endgroup$
– Mostafa Ayaz
Dec 2 '18 at 13:20
add a comment |
$begingroup$
We are told that $|a_n|^{frac1n}leq theta$ whenever $n geq n_0$. So if $n geq n_0$, it then follows that we have $0leq |a_n|leqtheta^n$. Since $theta in (0,1)$, it also follows that $sum_{n=1}^infty theta^n$ is a convergent geometric series. Now the comparison test seems relevant...
Always check to make sure you are using the hypotheses of the problem, otherwise things tend to go agley.
$endgroup$
add a comment |
$begingroup$
We are told that $|a_n|^{frac1n}leq theta$ whenever $n geq n_0$. So if $n geq n_0$, it then follows that we have $0leq |a_n|leqtheta^n$. Since $theta in (0,1)$, it also follows that $sum_{n=1}^infty theta^n$ is a convergent geometric series. Now the comparison test seems relevant...
Always check to make sure you are using the hypotheses of the problem, otherwise things tend to go agley.
$endgroup$
add a comment |
$begingroup$
We are told that $|a_n|^{frac1n}leq theta$ whenever $n geq n_0$. So if $n geq n_0$, it then follows that we have $0leq |a_n|leqtheta^n$. Since $theta in (0,1)$, it also follows that $sum_{n=1}^infty theta^n$ is a convergent geometric series. Now the comparison test seems relevant...
Always check to make sure you are using the hypotheses of the problem, otherwise things tend to go agley.
$endgroup$
We are told that $|a_n|^{frac1n}leq theta$ whenever $n geq n_0$. So if $n geq n_0$, it then follows that we have $0leq |a_n|leqtheta^n$. Since $theta in (0,1)$, it also follows that $sum_{n=1}^infty theta^n$ is a convergent geometric series. Now the comparison test seems relevant...
Always check to make sure you are using the hypotheses of the problem, otherwise things tend to go agley.
answered Dec 1 '18 at 21:36
Matt A PeltoMatt A Pelto
2,477620
2,477620
add a comment |
add a comment |
$begingroup$
$sqrt[n]{mid a_nmid}ltthetaimplies mid a_nmidletheta ^nimplies sum_{n=0}^infty mid a_nmidlesum_{n=0}^inftytheta^n=frac 1{1-theta}ltinfty $, since $theta lt1$.
Thus the series converges absolutely by comparison with a geometric series.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
add a comment |
$begingroup$
$sqrt[n]{mid a_nmid}ltthetaimplies mid a_nmidletheta ^nimplies sum_{n=0}^infty mid a_nmidlesum_{n=0}^inftytheta^n=frac 1{1-theta}ltinfty $, since $theta lt1$.
Thus the series converges absolutely by comparison with a geometric series.
$endgroup$
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
add a comment |
$begingroup$
$sqrt[n]{mid a_nmid}ltthetaimplies mid a_nmidletheta ^nimplies sum_{n=0}^infty mid a_nmidlesum_{n=0}^inftytheta^n=frac 1{1-theta}ltinfty $, since $theta lt1$.
Thus the series converges absolutely by comparison with a geometric series.
$endgroup$
$sqrt[n]{mid a_nmid}ltthetaimplies mid a_nmidletheta ^nimplies sum_{n=0}^infty mid a_nmidlesum_{n=0}^inftytheta^n=frac 1{1-theta}ltinfty $, since $theta lt1$.
Thus the series converges absolutely by comparison with a geometric series.
edited Dec 1 '18 at 22:14
answered Dec 1 '18 at 21:48
Chris CusterChris Custer
11.2k3824
11.2k3824
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
add a comment |
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
$begingroup$
How do you conclude that it equals $1/1-theta$ at the end?
$endgroup$
– SacredScout
Dec 1 '18 at 23:03
2
2
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
$begingroup$
$(1-theta)(1+theta+theta^2+dots+theta ^n)=1-theta ^{n+1}$. Divide both sides by $(1-theta) $ and let $ntoinfty$. This is known as "telescoping", because of all the cancellation.
$endgroup$
– Chris Custer
Dec 1 '18 at 23:25
add a comment |
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1
$begingroup$
$|a_n| le theta^n$
$endgroup$
– gammatester
Dec 1 '18 at 21:30
1
$begingroup$
I don't see why $0le |a_n| le theta <1$ implies the series converges. Anyway, the hypothesis is $sqrt[n]{a_n}le theta$, not $|a_n|le theta$.
$endgroup$
– Bernard
Dec 1 '18 at 21:32