Series interpretation of integral












0












$begingroup$


I'm currently stuck with the following question:



Prove, that $ln(2) = limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k}$ by rewriting the left side as an Integral.



So my current thoughts are:



$ln(2) = int_1^2 frac{1}{x} mathrm{d}x$



$limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k} = limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1} = limlimits_{n to 0} sum_{k=1}^{frac{2-1}{n}} n frac{1}{nk + 1}$



Like this, the sum is rewritten as a step function with the width of each equidistant step decreasing. If the sum is the upper sum, I have to show, that $frac{1}{nk+1}=ln(1 + frac{k+1}{n})$. How can I "get rid" if the $ln$?










share|cite|improve this question









$endgroup$












  • $begingroup$
    See math.stackexchange.com/questions/469885/…
    $endgroup$
    – lab bhattacharjee
    Feb 1 at 17:51
















0












$begingroup$


I'm currently stuck with the following question:



Prove, that $ln(2) = limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k}$ by rewriting the left side as an Integral.



So my current thoughts are:



$ln(2) = int_1^2 frac{1}{x} mathrm{d}x$



$limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k} = limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1} = limlimits_{n to 0} sum_{k=1}^{frac{2-1}{n}} n frac{1}{nk + 1}$



Like this, the sum is rewritten as a step function with the width of each equidistant step decreasing. If the sum is the upper sum, I have to show, that $frac{1}{nk+1}=ln(1 + frac{k+1}{n})$. How can I "get rid" if the $ln$?










share|cite|improve this question









$endgroup$












  • $begingroup$
    See math.stackexchange.com/questions/469885/…
    $endgroup$
    – lab bhattacharjee
    Feb 1 at 17:51














0












0








0





$begingroup$


I'm currently stuck with the following question:



Prove, that $ln(2) = limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k}$ by rewriting the left side as an Integral.



So my current thoughts are:



$ln(2) = int_1^2 frac{1}{x} mathrm{d}x$



$limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k} = limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1} = limlimits_{n to 0} sum_{k=1}^{frac{2-1}{n}} n frac{1}{nk + 1}$



Like this, the sum is rewritten as a step function with the width of each equidistant step decreasing. If the sum is the upper sum, I have to show, that $frac{1}{nk+1}=ln(1 + frac{k+1}{n})$. How can I "get rid" if the $ln$?










share|cite|improve this question









$endgroup$




I'm currently stuck with the following question:



Prove, that $ln(2) = limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k}$ by rewriting the left side as an Integral.



So my current thoughts are:



$ln(2) = int_1^2 frac{1}{x} mathrm{d}x$



$limlimits_{n to infty} sum_{k=1}^n frac{1}{n+k} = limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1} = limlimits_{n to 0} sum_{k=1}^{frac{2-1}{n}} n frac{1}{nk + 1}$



Like this, the sum is rewritten as a step function with the width of each equidistant step decreasing. If the sum is the upper sum, I have to show, that $frac{1}{nk+1}=ln(1 + frac{k+1}{n})$. How can I "get rid" if the $ln$?







real-analysis riemann-sum






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Feb 1 at 17:16









TimTim

246




246












  • $begingroup$
    See math.stackexchange.com/questions/469885/…
    $endgroup$
    – lab bhattacharjee
    Feb 1 at 17:51


















  • $begingroup$
    See math.stackexchange.com/questions/469885/…
    $endgroup$
    – lab bhattacharjee
    Feb 1 at 17:51
















$begingroup$
See math.stackexchange.com/questions/469885/…
$endgroup$
– lab bhattacharjee
Feb 1 at 17:51




$begingroup$
See math.stackexchange.com/questions/469885/…
$endgroup$
– lab bhattacharjee
Feb 1 at 17:51










2 Answers
2






active

oldest

votes


















1












$begingroup$

You're almost there, just use the approximation of the integral by a Riemann sum over the grid $1, 1+ 1/n,ldots, 2$, then



$$limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1}= limlimits_{n to infty} sum_{k=1}^n left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right] frac{1}{1+frac{k}{n}}=int_1^2frac{1}{x}dx.$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
    $endgroup$
    – Tim
    Feb 1 at 17:43










  • $begingroup$
    That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
    $endgroup$
    – Mars Plastic
    Feb 1 at 17:48












  • $begingroup$
    Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
    $endgroup$
    – Tim
    Feb 1 at 18:15










  • $begingroup$
    OK, so how is the integral introduced (defined) in your lecture?
    $endgroup$
    – Mars Plastic
    Feb 2 at 1:20










  • $begingroup$
    We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
    $endgroup$
    – Tim
    Feb 2 at 6:53





















1












$begingroup$

hint



Write it as



$$frac{b-a}{n}sum_{k=1}^nfleft(a+kfrac{b-a}{n}right)$$



the limit will be
$$int_a^bf(x)dx$$



In your case, $a=0; ; b=1, f(x)=frac{1}{1+x}$
Or
$a=1,;b=2 ;$ and$ ; f(x)=frac 1x.$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
    $endgroup$
    – Tim
    Feb 1 at 17:29














Your Answer





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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









1












$begingroup$

You're almost there, just use the approximation of the integral by a Riemann sum over the grid $1, 1+ 1/n,ldots, 2$, then



$$limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1}= limlimits_{n to infty} sum_{k=1}^n left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right] frac{1}{1+frac{k}{n}}=int_1^2frac{1}{x}dx.$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
    $endgroup$
    – Tim
    Feb 1 at 17:43










  • $begingroup$
    That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
    $endgroup$
    – Mars Plastic
    Feb 1 at 17:48












  • $begingroup$
    Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
    $endgroup$
    – Tim
    Feb 1 at 18:15










  • $begingroup$
    OK, so how is the integral introduced (defined) in your lecture?
    $endgroup$
    – Mars Plastic
    Feb 2 at 1:20










  • $begingroup$
    We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
    $endgroup$
    – Tim
    Feb 2 at 6:53


















1












$begingroup$

You're almost there, just use the approximation of the integral by a Riemann sum over the grid $1, 1+ 1/n,ldots, 2$, then



$$limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1}= limlimits_{n to infty} sum_{k=1}^n left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right] frac{1}{1+frac{k}{n}}=int_1^2frac{1}{x}dx.$$






share|cite|improve this answer









$endgroup$













  • $begingroup$
    So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
    $endgroup$
    – Tim
    Feb 1 at 17:43










  • $begingroup$
    That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
    $endgroup$
    – Mars Plastic
    Feb 1 at 17:48












  • $begingroup$
    Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
    $endgroup$
    – Tim
    Feb 1 at 18:15










  • $begingroup$
    OK, so how is the integral introduced (defined) in your lecture?
    $endgroup$
    – Mars Plastic
    Feb 2 at 1:20










  • $begingroup$
    We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
    $endgroup$
    – Tim
    Feb 2 at 6:53
















1












1








1





$begingroup$

You're almost there, just use the approximation of the integral by a Riemann sum over the grid $1, 1+ 1/n,ldots, 2$, then



$$limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1}= limlimits_{n to infty} sum_{k=1}^n left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right] frac{1}{1+frac{k}{n}}=int_1^2frac{1}{x}dx.$$






share|cite|improve this answer









$endgroup$



You're almost there, just use the approximation of the integral by a Riemann sum over the grid $1, 1+ 1/n,ldots, 2$, then



$$limlimits_{n to infty} sum_{k=1}^n frac{1}{n} frac{1}{frac{k}{n} + 1}= limlimits_{n to infty} sum_{k=1}^n left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right] frac{1}{1+frac{k}{n}}=int_1^2frac{1}{x}dx.$$







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Feb 1 at 17:28









Mars PlasticMars Plastic

1,455122




1,455122












  • $begingroup$
    So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
    $endgroup$
    – Tim
    Feb 1 at 17:43










  • $begingroup$
    That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
    $endgroup$
    – Mars Plastic
    Feb 1 at 17:48












  • $begingroup$
    Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
    $endgroup$
    – Tim
    Feb 1 at 18:15










  • $begingroup$
    OK, so how is the integral introduced (defined) in your lecture?
    $endgroup$
    – Mars Plastic
    Feb 2 at 1:20










  • $begingroup$
    We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
    $endgroup$
    – Tim
    Feb 2 at 6:53




















  • $begingroup$
    So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
    $endgroup$
    – Tim
    Feb 1 at 17:43










  • $begingroup$
    That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
    $endgroup$
    – Mars Plastic
    Feb 1 at 17:48












  • $begingroup$
    Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
    $endgroup$
    – Tim
    Feb 1 at 18:15










  • $begingroup$
    OK, so how is the integral introduced (defined) in your lecture?
    $endgroup$
    – Mars Plastic
    Feb 2 at 1:20










  • $begingroup$
    We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
    $endgroup$
    – Tim
    Feb 2 at 6:53


















$begingroup$
So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
$endgroup$
– Tim
Feb 1 at 17:43




$begingroup$
So $left[left(1+frac{k}{n}right)-left(1+frac{k-1}{n}right)right]$ is the "width" of the subdivision and $frac{1}{1+ frac{k}{n}}$ is the "height" respectively. But where do I know from, that this infinite sum is equal to the Integral?
$endgroup$
– Tim
Feb 1 at 17:43












$begingroup$
That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
$endgroup$
– Mars Plastic
Feb 1 at 17:48






$begingroup$
That's the definition of the Riemann integral. And be careful: It's not an infinite sum, but the limit of a sequence of finite sums.
$endgroup$
– Mars Plastic
Feb 1 at 17:48














$begingroup$
Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
$endgroup$
– Tim
Feb 1 at 18:15




$begingroup$
Ok, thank you very much. We haven't defined the Riemann sum in our lecture yet, is there a way of solving the problem without this definition?
$endgroup$
– Tim
Feb 1 at 18:15












$begingroup$
OK, so how is the integral introduced (defined) in your lecture?
$endgroup$
– Mars Plastic
Feb 2 at 1:20




$begingroup$
OK, so how is the integral introduced (defined) in your lecture?
$endgroup$
– Mars Plastic
Feb 2 at 1:20












$begingroup$
We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
$endgroup$
– Tim
Feb 2 at 6:53






$begingroup$
We have introduced the integral with step functions: Let $varphi$ be a step function, then the integral of $varphi$ over $[a,b]$ is defined as $int_a^b varphi(x) dx = sum_{i=1}^n c_i (x_i - x_{i-1})$. And we have the definition of the integral of a regulated function: If $(varphi_n)$ is uniformly converging to $f$, then $int_a^b f(x) dx = limlimits{n to infty}int_a^b varphi_n(x) dx$.
$endgroup$
– Tim
Feb 2 at 6:53













1












$begingroup$

hint



Write it as



$$frac{b-a}{n}sum_{k=1}^nfleft(a+kfrac{b-a}{n}right)$$



the limit will be
$$int_a^bf(x)dx$$



In your case, $a=0; ; b=1, f(x)=frac{1}{1+x}$
Or
$a=1,;b=2 ;$ and$ ; f(x)=frac 1x.$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
    $endgroup$
    – Tim
    Feb 1 at 17:29


















1












$begingroup$

hint



Write it as



$$frac{b-a}{n}sum_{k=1}^nfleft(a+kfrac{b-a}{n}right)$$



the limit will be
$$int_a^bf(x)dx$$



In your case, $a=0; ; b=1, f(x)=frac{1}{1+x}$
Or
$a=1,;b=2 ;$ and$ ; f(x)=frac 1x.$






share|cite|improve this answer











$endgroup$













  • $begingroup$
    So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
    $endgroup$
    – Tim
    Feb 1 at 17:29
















1












1








1





$begingroup$

hint



Write it as



$$frac{b-a}{n}sum_{k=1}^nfleft(a+kfrac{b-a}{n}right)$$



the limit will be
$$int_a^bf(x)dx$$



In your case, $a=0; ; b=1, f(x)=frac{1}{1+x}$
Or
$a=1,;b=2 ;$ and$ ; f(x)=frac 1x.$






share|cite|improve this answer











$endgroup$



hint



Write it as



$$frac{b-a}{n}sum_{k=1}^nfleft(a+kfrac{b-a}{n}right)$$



the limit will be
$$int_a^bf(x)dx$$



In your case, $a=0; ; b=1, f(x)=frac{1}{1+x}$
Or
$a=1,;b=2 ;$ and$ ; f(x)=frac 1x.$







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Feb 1 at 17:34









J.G.

33.2k23252




33.2k23252










answered Feb 1 at 17:21









hamam_Abdallahhamam_Abdallah

38.1k21634




38.1k21634












  • $begingroup$
    So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
    $endgroup$
    – Tim
    Feb 1 at 17:29




















  • $begingroup$
    So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
    $endgroup$
    – Tim
    Feb 1 at 17:29


















$begingroup$
So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
$endgroup$
– Tim
Feb 1 at 17:29






$begingroup$
So it would be $int_1^2 frac{1}{x} dx = frac{1}{n} sum_{k=1}^n frac{1}{1 + frac{k}{n}}$ which is equal to my upper transformation. But how can I prove the equivalence you provided to me?
$endgroup$
– Tim
Feb 1 at 17:29




















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