Convergence of $sum_{n=0}^{infty}(-2)^{n^2}/n!$ [closed]
Is this series convergent $$sum_{n=0}^inftyfrac{(-2)^{n^2}}{n!},?$$
I am supposed to find out if this series is convergent, absolutely convergent or divergent.
No test has given me information.
By this point i am aware that I'm supposed to simplify something but i don't know how. How do I get rid of $2^{n^2}$? Below is my attempt.
real-analysis sequences-and-series convergence
closed as off-topic by RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra Dec 7 at 7:59
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra
If this question can be reworded to fit the rules in the help center, please edit the question.
add a comment |
Is this series convergent $$sum_{n=0}^inftyfrac{(-2)^{n^2}}{n!},?$$
I am supposed to find out if this series is convergent, absolutely convergent or divergent.
No test has given me information.
By this point i am aware that I'm supposed to simplify something but i don't know how. How do I get rid of $2^{n^2}$? Below is my attempt.
real-analysis sequences-and-series convergence
closed as off-topic by RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra Dec 7 at 7:59
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra
If this question can be reworded to fit the rules in the help center, please edit the question.
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21
add a comment |
Is this series convergent $$sum_{n=0}^inftyfrac{(-2)^{n^2}}{n!},?$$
I am supposed to find out if this series is convergent, absolutely convergent or divergent.
No test has given me information.
By this point i am aware that I'm supposed to simplify something but i don't know how. How do I get rid of $2^{n^2}$? Below is my attempt.
real-analysis sequences-and-series convergence
Is this series convergent $$sum_{n=0}^inftyfrac{(-2)^{n^2}}{n!},?$$
I am supposed to find out if this series is convergent, absolutely convergent or divergent.
No test has given me information.
By this point i am aware that I'm supposed to simplify something but i don't know how. How do I get rid of $2^{n^2}$? Below is my attempt.
real-analysis sequences-and-series convergence
real-analysis sequences-and-series convergence
edited Dec 12 at 3:19
Batominovski
33.7k33292
33.7k33292
asked Dec 6 at 22:08
YoungDumbBroke
43
43
closed as off-topic by RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra Dec 7 at 7:59
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra
If this question can be reworded to fit the rules in the help center, please edit the question.
closed as off-topic by RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra Dec 7 at 7:59
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "This question is missing context or other details: Please provide additional context, which ideally explains why the question is relevant to you and our community. Some forms of context include: background and motivation, relevant definitions, source, possible strategies, your current progress, why the question is interesting or important, etc." – RRL, Leucippus, user10354138, ancientmathematician, Vidyanshu Mishra
If this question can be reworded to fit the rules in the help center, please edit the question.
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21
add a comment |
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21
add a comment |
4 Answers
4
active
oldest
votes
From $n!le n^n$ and $2^ngt n$ for all $nge1$, we have
$${2^{n^2}over n!}ge{2^{n^2}over n^n}=left(2^nover nright)^ngt1$$
so $(-2)^{n^2}/n!notto0$, and therefore the series diverges.
add a comment |
HINT
Let consider
$$lim_{nto infty}left|frac{(-2)^{n^2}}{n!}right|=lim_{nto infty}frac{2^{n^2}}{n!}$$
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
|
show 6 more comments
$|(-2)^{(n+1)^2}| = |2^{(n+1)^2}| = |2^{n^2 + 2n+ 1}| = (2^{n^2})(2^{2n})(2)$
Now apply the ratio test.
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
add a comment |
You seem to have an algebra error, using the Ratio Test, you have
$$
lim_{n to infty} left| dfrac{dfrac{(-2)^{(n+1)^2}}{(n+1)!}}{dfrac{(-2)^{n^2}}{n!}} right|= lim_{n to infty} dfrac{2^{n^2+2n+1}}{2^{n^2}} cdot dfrac{n!}{(n+1)!}= lim_{n to infty} dfrac{2^{n^2} cdot 2^{2n} cdot 2}{2^{n^2}} cdot dfrac{1}{n+1}= lim_{n to infty} dfrac{2^{2n+1}}{n+1}stackrel{L.H.}{=} lim_{n to infty} dfrac{2^{2n+1} ln 2 cdot 2}{1}= infty
$$
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
add a comment |
4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
From $n!le n^n$ and $2^ngt n$ for all $nge1$, we have
$${2^{n^2}over n!}ge{2^{n^2}over n^n}=left(2^nover nright)^ngt1$$
so $(-2)^{n^2}/n!notto0$, and therefore the series diverges.
add a comment |
From $n!le n^n$ and $2^ngt n$ for all $nge1$, we have
$${2^{n^2}over n!}ge{2^{n^2}over n^n}=left(2^nover nright)^ngt1$$
so $(-2)^{n^2}/n!notto0$, and therefore the series diverges.
add a comment |
From $n!le n^n$ and $2^ngt n$ for all $nge1$, we have
$${2^{n^2}over n!}ge{2^{n^2}over n^n}=left(2^nover nright)^ngt1$$
so $(-2)^{n^2}/n!notto0$, and therefore the series diverges.
From $n!le n^n$ and $2^ngt n$ for all $nge1$, we have
$${2^{n^2}over n!}ge{2^{n^2}over n^n}=left(2^nover nright)^ngt1$$
so $(-2)^{n^2}/n!notto0$, and therefore the series diverges.
answered Dec 6 at 23:20
Barry Cipra
58.8k653123
58.8k653123
add a comment |
add a comment |
HINT
Let consider
$$lim_{nto infty}left|frac{(-2)^{n^2}}{n!}right|=lim_{nto infty}frac{2^{n^2}}{n!}$$
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
|
show 6 more comments
HINT
Let consider
$$lim_{nto infty}left|frac{(-2)^{n^2}}{n!}right|=lim_{nto infty}frac{2^{n^2}}{n!}$$
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
|
show 6 more comments
HINT
Let consider
$$lim_{nto infty}left|frac{(-2)^{n^2}}{n!}right|=lim_{nto infty}frac{2^{n^2}}{n!}$$
HINT
Let consider
$$lim_{nto infty}left|frac{(-2)^{n^2}}{n!}right|=lim_{nto infty}frac{2^{n^2}}{n!}$$
edited Dec 6 at 22:17
answered Dec 6 at 22:11
gimusi
1
1
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
|
show 6 more comments
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
if the limit of this is not zero then the series diverges, right? I'm not sure how to solve for that limit though, can you be more specific?
– YoungDumbBroke
Dec 6 at 22:30
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
apply a log to the top and bottom And consider $ln n! = ln n + ln(n-1) + cdots < nln n$
– Doug M
Dec 6 at 22:34
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@YoungDumbBroke Exacly, try by ratio test!
– gimusi
Dec 6 at 22:35
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
@gimusi i tried but i get 1/0
– YoungDumbBroke
Dec 6 at 22:43
1
1
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
@YoungDumbBroke I've seen that but I don't get some step, we simply should have $$frac{2^{(n+1)^2}}{(n+1)!}frac{n!}{2^{n^2}}=frac{2^{(n+1)^2-n^2}}{n+1}=frac{2^{2n+1}}{n+1}$$
– gimusi
Dec 6 at 23:01
|
show 6 more comments
$|(-2)^{(n+1)^2}| = |2^{(n+1)^2}| = |2^{n^2 + 2n+ 1}| = (2^{n^2})(2^{2n})(2)$
Now apply the ratio test.
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
add a comment |
$|(-2)^{(n+1)^2}| = |2^{(n+1)^2}| = |2^{n^2 + 2n+ 1}| = (2^{n^2})(2^{2n})(2)$
Now apply the ratio test.
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
add a comment |
$|(-2)^{(n+1)^2}| = |2^{(n+1)^2}| = |2^{n^2 + 2n+ 1}| = (2^{n^2})(2^{2n})(2)$
Now apply the ratio test.
$|(-2)^{(n+1)^2}| = |2^{(n+1)^2}| = |2^{n^2 + 2n+ 1}| = (2^{n^2})(2^{2n})(2)$
Now apply the ratio test.
answered Dec 6 at 22:31
Doug M
43.9k31854
43.9k31854
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
add a comment |
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
why apply n+1 before doing the ratio test? I do get the problem solved that way but i don't understand why?
– YoungDumbBroke
Dec 6 at 22:40
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
The ratio test looks at $|frac{a_{n+1}}{a_n}|$ If the ratio is greater than 1 it does not converge (conditionally or absolutely). If the ratio is less than one it converges. If it equals 1 the test is inconclusive and may converge conditionally but not absolutely. I was just trying to show that $a_{n+1}$ can be manipulated to form something that is divisible by $a_n$
– Doug M
Dec 6 at 23:10
add a comment |
You seem to have an algebra error, using the Ratio Test, you have
$$
lim_{n to infty} left| dfrac{dfrac{(-2)^{(n+1)^2}}{(n+1)!}}{dfrac{(-2)^{n^2}}{n!}} right|= lim_{n to infty} dfrac{2^{n^2+2n+1}}{2^{n^2}} cdot dfrac{n!}{(n+1)!}= lim_{n to infty} dfrac{2^{n^2} cdot 2^{2n} cdot 2}{2^{n^2}} cdot dfrac{1}{n+1}= lim_{n to infty} dfrac{2^{2n+1}}{n+1}stackrel{L.H.}{=} lim_{n to infty} dfrac{2^{2n+1} ln 2 cdot 2}{1}= infty
$$
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
add a comment |
You seem to have an algebra error, using the Ratio Test, you have
$$
lim_{n to infty} left| dfrac{dfrac{(-2)^{(n+1)^2}}{(n+1)!}}{dfrac{(-2)^{n^2}}{n!}} right|= lim_{n to infty} dfrac{2^{n^2+2n+1}}{2^{n^2}} cdot dfrac{n!}{(n+1)!}= lim_{n to infty} dfrac{2^{n^2} cdot 2^{2n} cdot 2}{2^{n^2}} cdot dfrac{1}{n+1}= lim_{n to infty} dfrac{2^{2n+1}}{n+1}stackrel{L.H.}{=} lim_{n to infty} dfrac{2^{2n+1} ln 2 cdot 2}{1}= infty
$$
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
add a comment |
You seem to have an algebra error, using the Ratio Test, you have
$$
lim_{n to infty} left| dfrac{dfrac{(-2)^{(n+1)^2}}{(n+1)!}}{dfrac{(-2)^{n^2}}{n!}} right|= lim_{n to infty} dfrac{2^{n^2+2n+1}}{2^{n^2}} cdot dfrac{n!}{(n+1)!}= lim_{n to infty} dfrac{2^{n^2} cdot 2^{2n} cdot 2}{2^{n^2}} cdot dfrac{1}{n+1}= lim_{n to infty} dfrac{2^{2n+1}}{n+1}stackrel{L.H.}{=} lim_{n to infty} dfrac{2^{2n+1} ln 2 cdot 2}{1}= infty
$$
You seem to have an algebra error, using the Ratio Test, you have
$$
lim_{n to infty} left| dfrac{dfrac{(-2)^{(n+1)^2}}{(n+1)!}}{dfrac{(-2)^{n^2}}{n!}} right|= lim_{n to infty} dfrac{2^{n^2+2n+1}}{2^{n^2}} cdot dfrac{n!}{(n+1)!}= lim_{n to infty} dfrac{2^{n^2} cdot 2^{2n} cdot 2}{2^{n^2}} cdot dfrac{1}{n+1}= lim_{n to infty} dfrac{2^{2n+1}}{n+1}stackrel{L.H.}{=} lim_{n to infty} dfrac{2^{2n+1} ln 2 cdot 2}{1}= infty
$$
answered Dec 6 at 23:08
mathematics2x2life
8,04221738
8,04221738
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
add a comment |
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
we did not cover L.H. rule in class, so I can't use it
– YoungDumbBroke
Dec 6 at 23:12
add a comment |
Recall that we always need to check the foundamental necessary condition for any convergent $sum a_n$.
– gimusi
Dec 6 at 22:25
In order to reopen your OP, you should improve your question adding your trial by ratio test using MathJax. Do not hesitate to ask if you need some help. Bye
– gimusi
Dec 7 at 15:21