Convergence of the series $ sum_{n=1}^{+ infty } frac{1}{n cdot ln(n) cdot (ln(ln(n)))^{a}} $
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I have a problem with a convergence of the series $ sum_{n=1}^{+ infty } frac{1}{n cdot ln(n) cdot (ln(ln(n)))^{a}}$ depending on the parameter $a$. I know that $frac{1}{n} rightarrow 0$, $frac{1}{ln(n)} rightarrow 0$, but I don't know how to deal with $frac{1}{ln(ln(n)))^{a}}$ and how to get to the final answer. So far I solved tasks with $frac{1}{n ^{a}}$ and I know that $a>1 Leftrightarrow sum_{n=1}^{+ infty }frac{1}{n ^{a}}$ is convergent, but in this task I cannot use it. Can you help me?
real-analysis
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add a comment |
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I have a problem with a convergence of the series $ sum_{n=1}^{+ infty } frac{1}{n cdot ln(n) cdot (ln(ln(n)))^{a}}$ depending on the parameter $a$. I know that $frac{1}{n} rightarrow 0$, $frac{1}{ln(n)} rightarrow 0$, but I don't know how to deal with $frac{1}{ln(ln(n)))^{a}}$ and how to get to the final answer. So far I solved tasks with $frac{1}{n ^{a}}$ and I know that $a>1 Leftrightarrow sum_{n=1}^{+ infty }frac{1}{n ^{a}}$ is convergent, but in this task I cannot use it. Can you help me?
real-analysis
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add a comment |
$begingroup$
I have a problem with a convergence of the series $ sum_{n=1}^{+ infty } frac{1}{n cdot ln(n) cdot (ln(ln(n)))^{a}}$ depending on the parameter $a$. I know that $frac{1}{n} rightarrow 0$, $frac{1}{ln(n)} rightarrow 0$, but I don't know how to deal with $frac{1}{ln(ln(n)))^{a}}$ and how to get to the final answer. So far I solved tasks with $frac{1}{n ^{a}}$ and I know that $a>1 Leftrightarrow sum_{n=1}^{+ infty }frac{1}{n ^{a}}$ is convergent, but in this task I cannot use it. Can you help me?
real-analysis
$endgroup$
I have a problem with a convergence of the series $ sum_{n=1}^{+ infty } frac{1}{n cdot ln(n) cdot (ln(ln(n)))^{a}}$ depending on the parameter $a$. I know that $frac{1}{n} rightarrow 0$, $frac{1}{ln(n)} rightarrow 0$, but I don't know how to deal with $frac{1}{ln(ln(n)))^{a}}$ and how to get to the final answer. So far I solved tasks with $frac{1}{n ^{a}}$ and I know that $a>1 Leftrightarrow sum_{n=1}^{+ infty }frac{1}{n ^{a}}$ is convergent, but in this task I cannot use it. Can you help me?
real-analysis
real-analysis
edited Dec 13 '18 at 21:25
MP3129
asked Dec 13 '18 at 19:46
MP3129MP3129
36010
36010
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2 Answers
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This is set up very naturally for the integral test, after noting that the substitution $u = ln(ln x)$ leads to
$$int frac{1}{x ln x (ln ln x)^{a}} , dx = int frac 1 {u^a} , du.$$
The natural caveat, however, is that your series shouldn't start at $n = 1$ because $ln1 $ is a problem, and that $1/n to 0$ not $to 1$.
$endgroup$
add a comment |
$begingroup$
By Cauchy condensation test we can check the convergence by the condensed series $sum 2^na_{2^n}$ that is
$$sum_{n=1}^{ infty } frac{2^n}{2^n cdot ln(2^n) cdot (ln(ln(2^n)))^{a}}=sum_{n=1}^{ infty } frac{1}{ nln(2) cdot (ln (n ln 2))^{a}}$$
and then we can apply the test again to obtain
$$sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =$$$$=sum_{n=1}^{ infty }frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
and for the latter we can use limit comparison test with $sum_{n=1}^{ infty }frac1{n^a}$.
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Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
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– MP3129
Dec 13 '18 at 20:10
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@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
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– gimusi
Dec 13 '18 at 20:27
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But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
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@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
add a comment |
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2 Answers
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2 Answers
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$begingroup$
This is set up very naturally for the integral test, after noting that the substitution $u = ln(ln x)$ leads to
$$int frac{1}{x ln x (ln ln x)^{a}} , dx = int frac 1 {u^a} , du.$$
The natural caveat, however, is that your series shouldn't start at $n = 1$ because $ln1 $ is a problem, and that $1/n to 0$ not $to 1$.
$endgroup$
add a comment |
$begingroup$
This is set up very naturally for the integral test, after noting that the substitution $u = ln(ln x)$ leads to
$$int frac{1}{x ln x (ln ln x)^{a}} , dx = int frac 1 {u^a} , du.$$
The natural caveat, however, is that your series shouldn't start at $n = 1$ because $ln1 $ is a problem, and that $1/n to 0$ not $to 1$.
$endgroup$
add a comment |
$begingroup$
This is set up very naturally for the integral test, after noting that the substitution $u = ln(ln x)$ leads to
$$int frac{1}{x ln x (ln ln x)^{a}} , dx = int frac 1 {u^a} , du.$$
The natural caveat, however, is that your series shouldn't start at $n = 1$ because $ln1 $ is a problem, and that $1/n to 0$ not $to 1$.
$endgroup$
This is set up very naturally for the integral test, after noting that the substitution $u = ln(ln x)$ leads to
$$int frac{1}{x ln x (ln ln x)^{a}} , dx = int frac 1 {u^a} , du.$$
The natural caveat, however, is that your series shouldn't start at $n = 1$ because $ln1 $ is a problem, and that $1/n to 0$ not $to 1$.
answered Dec 13 '18 at 19:49
T. BongersT. Bongers
23.5k54762
23.5k54762
add a comment |
add a comment |
$begingroup$
By Cauchy condensation test we can check the convergence by the condensed series $sum 2^na_{2^n}$ that is
$$sum_{n=1}^{ infty } frac{2^n}{2^n cdot ln(2^n) cdot (ln(ln(2^n)))^{a}}=sum_{n=1}^{ infty } frac{1}{ nln(2) cdot (ln (n ln 2))^{a}}$$
and then we can apply the test again to obtain
$$sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =$$$$=sum_{n=1}^{ infty }frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
and for the latter we can use limit comparison test with $sum_{n=1}^{ infty }frac1{n^a}$.
$endgroup$
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
add a comment |
$begingroup$
By Cauchy condensation test we can check the convergence by the condensed series $sum 2^na_{2^n}$ that is
$$sum_{n=1}^{ infty } frac{2^n}{2^n cdot ln(2^n) cdot (ln(ln(2^n)))^{a}}=sum_{n=1}^{ infty } frac{1}{ nln(2) cdot (ln (n ln 2))^{a}}$$
and then we can apply the test again to obtain
$$sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =$$$$=sum_{n=1}^{ infty }frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
and for the latter we can use limit comparison test with $sum_{n=1}^{ infty }frac1{n^a}$.
$endgroup$
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
add a comment |
$begingroup$
By Cauchy condensation test we can check the convergence by the condensed series $sum 2^na_{2^n}$ that is
$$sum_{n=1}^{ infty } frac{2^n}{2^n cdot ln(2^n) cdot (ln(ln(2^n)))^{a}}=sum_{n=1}^{ infty } frac{1}{ nln(2) cdot (ln (n ln 2))^{a}}$$
and then we can apply the test again to obtain
$$sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =$$$$=sum_{n=1}^{ infty }frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
and for the latter we can use limit comparison test with $sum_{n=1}^{ infty }frac1{n^a}$.
$endgroup$
By Cauchy condensation test we can check the convergence by the condensed series $sum 2^na_{2^n}$ that is
$$sum_{n=1}^{ infty } frac{2^n}{2^n cdot ln(2^n) cdot (ln(ln(2^n)))^{a}}=sum_{n=1}^{ infty } frac{1}{ nln(2) cdot (ln (n ln 2))^{a}}$$
and then we can apply the test again to obtain
$$sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =sum_{n=1}^{ infty }frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =$$$$=sum_{n=1}^{ infty }frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
and for the latter we can use limit comparison test with $sum_{n=1}^{ infty }frac1{n^a}$.
edited Dec 14 '18 at 8:29
answered Dec 13 '18 at 19:49
gimusigimusi
92.9k84494
92.9k84494
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
add a comment |
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
Ok, so I have a $frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}}$ and $ln(2) rightarrow ln(2) $, więc we have to deal only $(ln(2^{n}ln(2))^{a}$, but I still do not know how to do it
$endgroup$
– MP3129
Dec 13 '18 at 20:10
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
@MP3129 You are almost done indeed $$frac{1}{ln(2)cdot(ln(2^{n}ln(2))^{a}} =frac{1}{ln(2)cdot(ln(2^{n})+ln(ln(2)) )^{a}} =frac{1}{ln(2)cdot n^a(ln(2)+ln(ln(2))/n )^{a}}$$
$endgroup$
– gimusi
Dec 13 '18 at 20:27
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
But I don't know what I can do with $(ln(2)+ln(ln(2))/n)^{a}$
$endgroup$
– MP3129
Dec 13 '18 at 21:05
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
$begingroup$
@MP3129 Simply note that $(ln(2)+ln(ln(2))/n)^{a}to (ln 2)^a$ then for the series we can refer to limit comparison test with $sum frac1{n^a}$.
$endgroup$
– gimusi
Dec 13 '18 at 21:07
add a comment |
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