Limit, Riemann Sum, Integration, Natural logarithm
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For any natural number $m$, $lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=ln (m)$.
I tried to prove the statement in the following way.
Proof:
$$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$
Dividing the numerator and the denominator of $frac{1}{n+r}$ by $n$, we get $frac{1/n}{1+r/n}$.
Therefore,
$$lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}=lim_{nrightarrow infty }frac{1}{n}sum_{r=1}^{(m-1)n}frac{1}{1+r/n}$$
this is a Riemann sum, so replacing $frac{1}{n}$ with $dx$, $frac{r}{n}$ with $x$, and integrating between the limits $x=0$ and $x=m-1$
we get $
$$int_{0}^{m-1}frac{dx}{1+x}=ln(1+x)|_{0}^{m-1}=ln(m)-ln(1)=ln(m)blacksquare$$
Is this a valid way?
integration limits logarithms riemann-integration riemann-sum
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add a comment |
$begingroup$
For any natural number $m$, $lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=ln (m)$.
I tried to prove the statement in the following way.
Proof:
$$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$
Dividing the numerator and the denominator of $frac{1}{n+r}$ by $n$, we get $frac{1/n}{1+r/n}$.
Therefore,
$$lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}=lim_{nrightarrow infty }frac{1}{n}sum_{r=1}^{(m-1)n}frac{1}{1+r/n}$$
this is a Riemann sum, so replacing $frac{1}{n}$ with $dx$, $frac{r}{n}$ with $x$, and integrating between the limits $x=0$ and $x=m-1$
we get $
$$int_{0}^{m-1}frac{dx}{1+x}=ln(1+x)|_{0}^{m-1}=ln(m)-ln(1)=ln(m)blacksquare$$
Is this a valid way?
integration limits logarithms riemann-integration riemann-sum
$endgroup$
$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
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– gimusi
Dec 4 '18 at 8:38
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@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47
add a comment |
$begingroup$
For any natural number $m$, $lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=ln (m)$.
I tried to prove the statement in the following way.
Proof:
$$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$
Dividing the numerator and the denominator of $frac{1}{n+r}$ by $n$, we get $frac{1/n}{1+r/n}$.
Therefore,
$$lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}=lim_{nrightarrow infty }frac{1}{n}sum_{r=1}^{(m-1)n}frac{1}{1+r/n}$$
this is a Riemann sum, so replacing $frac{1}{n}$ with $dx$, $frac{r}{n}$ with $x$, and integrating between the limits $x=0$ and $x=m-1$
we get $
$$int_{0}^{m-1}frac{dx}{1+x}=ln(1+x)|_{0}^{m-1}=ln(m)-ln(1)=ln(m)blacksquare$$
Is this a valid way?
integration limits logarithms riemann-integration riemann-sum
$endgroup$
For any natural number $m$, $lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=ln (m)$.
I tried to prove the statement in the following way.
Proof:
$$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{mn} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$
Dividing the numerator and the denominator of $frac{1}{n+r}$ by $n$, we get $frac{1/n}{1+r/n}$.
Therefore,
$$lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}=lim_{nrightarrow infty }frac{1}{n}sum_{r=1}^{(m-1)n}frac{1}{1+r/n}$$
this is a Riemann sum, so replacing $frac{1}{n}$ with $dx$, $frac{r}{n}$ with $x$, and integrating between the limits $x=0$ and $x=m-1$
we get $
$$int_{0}^{m-1}frac{dx}{1+x}=ln(1+x)|_{0}^{m-1}=ln(m)-ln(1)=ln(m)blacksquare$$
Is this a valid way?
integration limits logarithms riemann-integration riemann-sum
integration limits logarithms riemann-integration riemann-sum
edited Dec 4 '18 at 10:45
gimusi
92.8k84494
92.8k84494
asked Dec 4 '18 at 8:16
Hussain-AlqatariHussain-Alqatari
3187
3187
$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
$endgroup$
– gimusi
Dec 4 '18 at 8:38
$begingroup$
@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47
add a comment |
$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
$endgroup$
– gimusi
Dec 4 '18 at 8:38
$begingroup$
@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47
$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
$endgroup$
– gimusi
Dec 4 '18 at 8:38
$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
$endgroup$
– gimusi
Dec 4 '18 at 8:38
$begingroup$
@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47
add a comment |
2 Answers
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Yes your method is correct.
As an alternative by harmonic series
$$sum_{r=1}^{(m-1)n}frac{1}{n+r}=sum_{r=1}^{mn}frac{1}{r}-sum_{r=1}^{n}frac{1}{r}sim ln(mn)-ln(n)=ln m$$
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Can you please check again?
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– Hussain-Alqatari
Dec 4 '18 at 8:50
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Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
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@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
add a comment |
$begingroup$
A simpler and more direct choice is to write
$$lim_{n to infty} left( frac{1}{n+1} + frac{1}{n+2} + cdots + frac{1}{mn} right) = lim_{n to infty} sum_{r=n+1}^{mn} frac{1}{r} = lim_{n to infty} frac{1}{n} sum_{r=n+1}^{mn} frac{1}{r/n} = int_{x=1}^m frac{1}{x} , dx = log m,$$ but your solution is valid.
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Many Thanks heropup!
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– Hussain-Alqatari
Dec 5 '18 at 15:22
add a comment |
Your Answer
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2 Answers
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2 Answers
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$begingroup$
Yes your method is correct.
As an alternative by harmonic series
$$sum_{r=1}^{(m-1)n}frac{1}{n+r}=sum_{r=1}^{mn}frac{1}{r}-sum_{r=1}^{n}frac{1}{r}sim ln(mn)-ln(n)=ln m$$
$endgroup$
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
add a comment |
$begingroup$
Yes your method is correct.
As an alternative by harmonic series
$$sum_{r=1}^{(m-1)n}frac{1}{n+r}=sum_{r=1}^{mn}frac{1}{r}-sum_{r=1}^{n}frac{1}{r}sim ln(mn)-ln(n)=ln m$$
$endgroup$
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
add a comment |
$begingroup$
Yes your method is correct.
As an alternative by harmonic series
$$sum_{r=1}^{(m-1)n}frac{1}{n+r}=sum_{r=1}^{mn}frac{1}{r}-sum_{r=1}^{n}frac{1}{r}sim ln(mn)-ln(n)=ln m$$
$endgroup$
Yes your method is correct.
As an alternative by harmonic series
$$sum_{r=1}^{(m-1)n}frac{1}{n+r}=sum_{r=1}^{mn}frac{1}{r}-sum_{r=1}^{n}frac{1}{r}sim ln(mn)-ln(n)=ln m$$
edited Dec 4 '18 at 8:51
answered Dec 4 '18 at 8:21
gimusigimusi
92.8k84494
92.8k84494
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
add a comment |
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Can you please check again?
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:50
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
Thanks dear gimusi
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:25
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
$begingroup$
@Hussain-Alqatari You are welcome and you did a great work there! Bye
$endgroup$
– gimusi
Dec 5 '18 at 15:27
add a comment |
$begingroup$
A simpler and more direct choice is to write
$$lim_{n to infty} left( frac{1}{n+1} + frac{1}{n+2} + cdots + frac{1}{mn} right) = lim_{n to infty} sum_{r=n+1}^{mn} frac{1}{r} = lim_{n to infty} frac{1}{n} sum_{r=n+1}^{mn} frac{1}{r/n} = int_{x=1}^m frac{1}{x} , dx = log m,$$ but your solution is valid.
$endgroup$
$begingroup$
Many Thanks heropup!
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:22
add a comment |
$begingroup$
A simpler and more direct choice is to write
$$lim_{n to infty} left( frac{1}{n+1} + frac{1}{n+2} + cdots + frac{1}{mn} right) = lim_{n to infty} sum_{r=n+1}^{mn} frac{1}{r} = lim_{n to infty} frac{1}{n} sum_{r=n+1}^{mn} frac{1}{r/n} = int_{x=1}^m frac{1}{x} , dx = log m,$$ but your solution is valid.
$endgroup$
$begingroup$
Many Thanks heropup!
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:22
add a comment |
$begingroup$
A simpler and more direct choice is to write
$$lim_{n to infty} left( frac{1}{n+1} + frac{1}{n+2} + cdots + frac{1}{mn} right) = lim_{n to infty} sum_{r=n+1}^{mn} frac{1}{r} = lim_{n to infty} frac{1}{n} sum_{r=n+1}^{mn} frac{1}{r/n} = int_{x=1}^m frac{1}{x} , dx = log m,$$ but your solution is valid.
$endgroup$
A simpler and more direct choice is to write
$$lim_{n to infty} left( frac{1}{n+1} + frac{1}{n+2} + cdots + frac{1}{mn} right) = lim_{n to infty} sum_{r=n+1}^{mn} frac{1}{r} = lim_{n to infty} frac{1}{n} sum_{r=n+1}^{mn} frac{1}{r/n} = int_{x=1}^m frac{1}{x} , dx = log m,$$ but your solution is valid.
answered Dec 5 '18 at 10:17
heropupheropup
63k66199
63k66199
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Many Thanks heropup!
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– Hussain-Alqatari
Dec 5 '18 at 15:22
add a comment |
$begingroup$
Many Thanks heropup!
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:22
$begingroup$
Many Thanks heropup!
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:22
$begingroup$
Many Thanks heropup!
$endgroup$
– Hussain-Alqatari
Dec 5 '18 at 15:22
add a comment |
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$begingroup$
Why $$lim_{nrightarrow infty }left ( frac{1}{n+1}+frac{1}{n+2}+frac{1}{n+3}+cdots +frac{1}{m+n} right )=lim_{nrightarrow infty }sum_{r=1}^{(m-1)n}frac{1}{n+r}$$ it should be $$=lim_{nrightarrow infty }sum_{r=1}^{m}frac{1}{n+r}$$
$endgroup$
– gimusi
Dec 4 '18 at 8:38
$begingroup$
@gimusi Oops! The denominator of the last term in the expression of the limit should be $mn$ instead of $m+n$.
$endgroup$
– Hussain-Alqatari
Dec 4 '18 at 8:46
$begingroup$
Ah ok! I revise my answer accordingly!
$endgroup$
– gimusi
Dec 4 '18 at 8:47