Determine whether the series $frac{e^frac{1}{n}}{n^2}$ converges or diverges












0














The problem states




Determine whether the series converges or diverges: $$sum_{n=1}^inftyfrac{e^{1/n}}{n^2}$$




Immediately I always like to try the test for divergence:



$$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{1/n}}{n^2} = 0$$ I judge this to be true by looking at the numerator and denominator of the series. The numerator will tend to 1 as $nto infty$ and the denominator will grow bigger and bigger, so eventually you will have $frac{1}{infty}$ which will equal to zero. This tells us absolutely nothing about the convergence or divergence of this series.



Next I'll try the integral test because i see in the denominator a term resulting from the derivative of the numerator. To apply the integral test the sequence $a_n$ of our series $sum_{n=1}^infty a_n$ should be continuous, positive and decreasing. The summation begins at one and not zero so we have no issues with continuity here. Both numerator and denominator will yield positive terms as $n to infty$ and sequential terms will get smaller as the numerator shrinks whilst the denominator grows as $nto infty$.



$$text{let } f(x) = frac{e^{x^{-1}}}{x^2}$$
$$int_1^infty f(x) dx = int_1^infty frac{e^{x^{-1}}}{x^2}dx$$
$$= -e^{x{^{-1}}}|_1^infty$$
$$= lim_{xto infty}- e^{frac{1}{x}} +e$$
$$=-1 + e$$



because $int_1^infty f(x)$ converges then by the integral test so must our series.



I have two questions:




  1. How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?

  2. Would it be possible to determine whether this series converges or diverges using the limit comparison test?










share|cite|improve this question




















  • 3




    Compare, to say $e/n^2$.
    – Lord Shark the Unknown
    Oct 1 '17 at 16:19










  • This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:27










  • The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
    – Blargian
    Oct 1 '17 at 16:49










  • Yes, that is correct.
    – Simply Beautiful Art
    Oct 1 '17 at 16:53










  • Don’t forget the $dx$ on your integral
    – Chase Ryan Taylor
    Oct 1 '17 at 18:24
















0














The problem states




Determine whether the series converges or diverges: $$sum_{n=1}^inftyfrac{e^{1/n}}{n^2}$$




Immediately I always like to try the test for divergence:



$$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{1/n}}{n^2} = 0$$ I judge this to be true by looking at the numerator and denominator of the series. The numerator will tend to 1 as $nto infty$ and the denominator will grow bigger and bigger, so eventually you will have $frac{1}{infty}$ which will equal to zero. This tells us absolutely nothing about the convergence or divergence of this series.



Next I'll try the integral test because i see in the denominator a term resulting from the derivative of the numerator. To apply the integral test the sequence $a_n$ of our series $sum_{n=1}^infty a_n$ should be continuous, positive and decreasing. The summation begins at one and not zero so we have no issues with continuity here. Both numerator and denominator will yield positive terms as $n to infty$ and sequential terms will get smaller as the numerator shrinks whilst the denominator grows as $nto infty$.



$$text{let } f(x) = frac{e^{x^{-1}}}{x^2}$$
$$int_1^infty f(x) dx = int_1^infty frac{e^{x^{-1}}}{x^2}dx$$
$$= -e^{x{^{-1}}}|_1^infty$$
$$= lim_{xto infty}- e^{frac{1}{x}} +e$$
$$=-1 + e$$



because $int_1^infty f(x)$ converges then by the integral test so must our series.



I have two questions:




  1. How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?

  2. Would it be possible to determine whether this series converges or diverges using the limit comparison test?










share|cite|improve this question




















  • 3




    Compare, to say $e/n^2$.
    – Lord Shark the Unknown
    Oct 1 '17 at 16:19










  • This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:27










  • The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
    – Blargian
    Oct 1 '17 at 16:49










  • Yes, that is correct.
    – Simply Beautiful Art
    Oct 1 '17 at 16:53










  • Don’t forget the $dx$ on your integral
    – Chase Ryan Taylor
    Oct 1 '17 at 18:24














0












0








0


1





The problem states




Determine whether the series converges or diverges: $$sum_{n=1}^inftyfrac{e^{1/n}}{n^2}$$




Immediately I always like to try the test for divergence:



$$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{1/n}}{n^2} = 0$$ I judge this to be true by looking at the numerator and denominator of the series. The numerator will tend to 1 as $nto infty$ and the denominator will grow bigger and bigger, so eventually you will have $frac{1}{infty}$ which will equal to zero. This tells us absolutely nothing about the convergence or divergence of this series.



Next I'll try the integral test because i see in the denominator a term resulting from the derivative of the numerator. To apply the integral test the sequence $a_n$ of our series $sum_{n=1}^infty a_n$ should be continuous, positive and decreasing. The summation begins at one and not zero so we have no issues with continuity here. Both numerator and denominator will yield positive terms as $n to infty$ and sequential terms will get smaller as the numerator shrinks whilst the denominator grows as $nto infty$.



$$text{let } f(x) = frac{e^{x^{-1}}}{x^2}$$
$$int_1^infty f(x) dx = int_1^infty frac{e^{x^{-1}}}{x^2}dx$$
$$= -e^{x{^{-1}}}|_1^infty$$
$$= lim_{xto infty}- e^{frac{1}{x}} +e$$
$$=-1 + e$$



because $int_1^infty f(x)$ converges then by the integral test so must our series.



I have two questions:




  1. How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?

  2. Would it be possible to determine whether this series converges or diverges using the limit comparison test?










share|cite|improve this question















The problem states




Determine whether the series converges or diverges: $$sum_{n=1}^inftyfrac{e^{1/n}}{n^2}$$




Immediately I always like to try the test for divergence:



$$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{1/n}}{n^2} = 0$$ I judge this to be true by looking at the numerator and denominator of the series. The numerator will tend to 1 as $nto infty$ and the denominator will grow bigger and bigger, so eventually you will have $frac{1}{infty}$ which will equal to zero. This tells us absolutely nothing about the convergence or divergence of this series.



Next I'll try the integral test because i see in the denominator a term resulting from the derivative of the numerator. To apply the integral test the sequence $a_n$ of our series $sum_{n=1}^infty a_n$ should be continuous, positive and decreasing. The summation begins at one and not zero so we have no issues with continuity here. Both numerator and denominator will yield positive terms as $n to infty$ and sequential terms will get smaller as the numerator shrinks whilst the denominator grows as $nto infty$.



$$text{let } f(x) = frac{e^{x^{-1}}}{x^2}$$
$$int_1^infty f(x) dx = int_1^infty frac{e^{x^{-1}}}{x^2}dx$$
$$= -e^{x{^{-1}}}|_1^infty$$
$$= lim_{xto infty}- e^{frac{1}{x}} +e$$
$$=-1 + e$$



because $int_1^infty f(x)$ converges then by the integral test so must our series.



I have two questions:




  1. How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?

  2. Would it be possible to determine whether this series converges or diverges using the limit comparison test?







sequences-and-series convergence






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edited Nov 25 at 11:35









amWhy

191k28224439




191k28224439










asked Oct 1 '17 at 16:16









Blargian

161314




161314








  • 3




    Compare, to say $e/n^2$.
    – Lord Shark the Unknown
    Oct 1 '17 at 16:19










  • This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:27










  • The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
    – Blargian
    Oct 1 '17 at 16:49










  • Yes, that is correct.
    – Simply Beautiful Art
    Oct 1 '17 at 16:53










  • Don’t forget the $dx$ on your integral
    – Chase Ryan Taylor
    Oct 1 '17 at 18:24














  • 3




    Compare, to say $e/n^2$.
    – Lord Shark the Unknown
    Oct 1 '17 at 16:19










  • This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:27










  • The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
    – Blargian
    Oct 1 '17 at 16:49










  • Yes, that is correct.
    – Simply Beautiful Art
    Oct 1 '17 at 16:53










  • Don’t forget the $dx$ on your integral
    – Chase Ryan Taylor
    Oct 1 '17 at 18:24








3




3




Compare, to say $e/n^2$.
– Lord Shark the Unknown
Oct 1 '17 at 16:19




Compare, to say $e/n^2$.
– Lord Shark the Unknown
Oct 1 '17 at 16:19












This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
– Simply Beautiful Art
Oct 1 '17 at 16:27




This makes no sense: $$lim_{ntoinfty} sum_{n=1}^inftyfrac{e^{frac{1}{n}}}{n^2} = 0$$
– Simply Beautiful Art
Oct 1 '17 at 16:27












The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
– Blargian
Oct 1 '17 at 16:49




The test for divergence is applied to the sequence and not the series? I.e) if i had written $$ lim_{ntoinfty} frac{e^{frac{1}{n}}}{n^2} = 0$$ that would be fine?
– Blargian
Oct 1 '17 at 16:49












Yes, that is correct.
– Simply Beautiful Art
Oct 1 '17 at 16:53




Yes, that is correct.
– Simply Beautiful Art
Oct 1 '17 at 16:53












Don’t forget the $dx$ on your integral
– Chase Ryan Taylor
Oct 1 '17 at 18:24




Don’t forget the $dx$ on your integral
– Chase Ryan Taylor
Oct 1 '17 at 18:24










1 Answer
1






active

oldest

votes


















1














As Lord Shark the Unknown comments, simply note that



$$e^{1/n}<e,quad n>1$$



And so,



$$0<sum_{n=1}^inftyfrac{e^{1/n}}{n^2}<sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6<infty$$




How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?




If $f(x)$ is continuous, $f(x)ge0$, and $f'(x)le0$, then...




Would it be possible to determine whether this series converges or diverges using the limit comparison test?




Sure. You could apply the limit comparison test against $1/n^2$, since $e^{1/n}to1$ as $ntoinfty$.






share|cite|improve this answer





















  • Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
    – Blargian
    Oct 1 '17 at 16:50






  • 1




    See the Basel problem.
    – Simply Beautiful Art
    Oct 1 '17 at 16:52






  • 1




    You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:56






  • 1




    If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
    – Simply Beautiful Art
    Oct 1 '17 at 16:57











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1 Answer
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oldest

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1














As Lord Shark the Unknown comments, simply note that



$$e^{1/n}<e,quad n>1$$



And so,



$$0<sum_{n=1}^inftyfrac{e^{1/n}}{n^2}<sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6<infty$$




How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?




If $f(x)$ is continuous, $f(x)ge0$, and $f'(x)le0$, then...




Would it be possible to determine whether this series converges or diverges using the limit comparison test?




Sure. You could apply the limit comparison test against $1/n^2$, since $e^{1/n}to1$ as $ntoinfty$.






share|cite|improve this answer





















  • Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
    – Blargian
    Oct 1 '17 at 16:50






  • 1




    See the Basel problem.
    – Simply Beautiful Art
    Oct 1 '17 at 16:52






  • 1




    You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:56






  • 1




    If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
    – Simply Beautiful Art
    Oct 1 '17 at 16:57
















1














As Lord Shark the Unknown comments, simply note that



$$e^{1/n}<e,quad n>1$$



And so,



$$0<sum_{n=1}^inftyfrac{e^{1/n}}{n^2}<sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6<infty$$




How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?




If $f(x)$ is continuous, $f(x)ge0$, and $f'(x)le0$, then...




Would it be possible to determine whether this series converges or diverges using the limit comparison test?




Sure. You could apply the limit comparison test against $1/n^2$, since $e^{1/n}to1$ as $ntoinfty$.






share|cite|improve this answer





















  • Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
    – Blargian
    Oct 1 '17 at 16:50






  • 1




    See the Basel problem.
    – Simply Beautiful Art
    Oct 1 '17 at 16:52






  • 1




    You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:56






  • 1




    If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
    – Simply Beautiful Art
    Oct 1 '17 at 16:57














1












1








1






As Lord Shark the Unknown comments, simply note that



$$e^{1/n}<e,quad n>1$$



And so,



$$0<sum_{n=1}^inftyfrac{e^{1/n}}{n^2}<sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6<infty$$




How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?




If $f(x)$ is continuous, $f(x)ge0$, and $f'(x)le0$, then...




Would it be possible to determine whether this series converges or diverges using the limit comparison test?




Sure. You could apply the limit comparison test against $1/n^2$, since $e^{1/n}to1$ as $ntoinfty$.






share|cite|improve this answer












As Lord Shark the Unknown comments, simply note that



$$e^{1/n}<e,quad n>1$$



And so,



$$0<sum_{n=1}^inftyfrac{e^{1/n}}{n^2}<sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6<infty$$




How can i write the conditions for the integral test (continuous,positive,decreasing) in a mathematical way. What would be the mathematically accepted way to show these conditions?




If $f(x)$ is continuous, $f(x)ge0$, and $f'(x)le0$, then...




Would it be possible to determine whether this series converges or diverges using the limit comparison test?




Sure. You could apply the limit comparison test against $1/n^2$, since $e^{1/n}to1$ as $ntoinfty$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Oct 1 '17 at 16:31









Simply Beautiful Art

50.2k578181




50.2k578181












  • Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
    – Blargian
    Oct 1 '17 at 16:50






  • 1




    See the Basel problem.
    – Simply Beautiful Art
    Oct 1 '17 at 16:52






  • 1




    You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:56






  • 1




    If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
    – Simply Beautiful Art
    Oct 1 '17 at 16:57


















  • Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
    – Blargian
    Oct 1 '17 at 16:50






  • 1




    See the Basel problem.
    – Simply Beautiful Art
    Oct 1 '17 at 16:52






  • 1




    You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
    – Simply Beautiful Art
    Oct 1 '17 at 16:56






  • 1




    If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
    – Simply Beautiful Art
    Oct 1 '17 at 16:57
















Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
– Blargian
Oct 1 '17 at 16:50




Could you please explain how $$sum_{n=1}^inftyfrac e{n^2}=frac{epi^2}6$$
– Blargian
Oct 1 '17 at 16:50




1




1




See the Basel problem.
– Simply Beautiful Art
Oct 1 '17 at 16:52




See the Basel problem.
– Simply Beautiful Art
Oct 1 '17 at 16:52




1




1




You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
– Simply Beautiful Art
Oct 1 '17 at 16:56




You probably don't want this, but another interpretation of the conditions for the integral test would be$$(fin C^1)landforall xge0[(f(x)ge0)land f'(x)le0)]impliesleft(sum_{n=0}^infty f(n)<inftyiffint_0^infty f(x)~mathrm dx<inftyright)$$
– Simply Beautiful Art
Oct 1 '17 at 16:56




1




1




If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
– Simply Beautiful Art
Oct 1 '17 at 16:57




If you don't want to restrict this to differentiable functions, then use $forall xge0forall yge0[x<yimplies f(x)>f(y)]$
– Simply Beautiful Art
Oct 1 '17 at 16:57


















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