Conversion of $int_0^1fraclog( 1+x)xdx$ to summation

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We know,




$$lim_ntoinftyfrac1nsum_r=1^nfleft(frac rnright)=int_0^1f(x)dx$$




Now,we have $int_0^1fraclog( 1+x)xdx$
So,$f(x)=fraclog (1+x)x$.So we find $f(r/n)$ by replacing $x$ with $r/n$. Then putting in the summation form on LHS.



Converting to summation,
$$lim_ntoinftysum_r=1^nfraclogleft(1+fracrnright)r$$



Now this should evaluate to $1-frac12^2+frac13^3-cdots$.How do I carry on the simplification?



Thanks for any help!










share|cite|improve this question























  • Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
    – Aditya
    Sep 2 at 8:06














up vote
1
down vote

favorite












We know,




$$lim_ntoinftyfrac1nsum_r=1^nfleft(frac rnright)=int_0^1f(x)dx$$




Now,we have $int_0^1fraclog( 1+x)xdx$
So,$f(x)=fraclog (1+x)x$.So we find $f(r/n)$ by replacing $x$ with $r/n$. Then putting in the summation form on LHS.



Converting to summation,
$$lim_ntoinftysum_r=1^nfraclogleft(1+fracrnright)r$$



Now this should evaluate to $1-frac12^2+frac13^3-cdots$.How do I carry on the simplification?



Thanks for any help!










share|cite|improve this question























  • Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
    – Aditya
    Sep 2 at 8:06












up vote
1
down vote

favorite









up vote
1
down vote

favorite











We know,




$$lim_ntoinftyfrac1nsum_r=1^nfleft(frac rnright)=int_0^1f(x)dx$$




Now,we have $int_0^1fraclog( 1+x)xdx$
So,$f(x)=fraclog (1+x)x$.So we find $f(r/n)$ by replacing $x$ with $r/n$. Then putting in the summation form on LHS.



Converting to summation,
$$lim_ntoinftysum_r=1^nfraclogleft(1+fracrnright)r$$



Now this should evaluate to $1-frac12^2+frac13^3-cdots$.How do I carry on the simplification?



Thanks for any help!










share|cite|improve this question















We know,




$$lim_ntoinftyfrac1nsum_r=1^nfleft(frac rnright)=int_0^1f(x)dx$$




Now,we have $int_0^1fraclog( 1+x)xdx$
So,$f(x)=fraclog (1+x)x$.So we find $f(r/n)$ by replacing $x$ with $r/n$. Then putting in the summation form on LHS.



Converting to summation,
$$lim_ntoinftysum_r=1^nfraclogleft(1+fracrnright)r$$



Now this should evaluate to $1-frac12^2+frac13^3-cdots$.How do I carry on the simplification?



Thanks for any help!







calculus integration limits summation






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edited Sep 3 at 11:53









Batominovski

25.7k22881




25.7k22881










asked Sep 2 at 7:55









tatan

5,13542355




5,13542355











  • Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
    – Aditya
    Sep 2 at 8:06
















  • Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
    – Aditya
    Sep 2 at 8:06















Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
– Aditya
Sep 2 at 8:06




Assuming $log(x)$ here is a natural log ( or it can be converted to one by appropriately scaling it). You can use the taylor series approximation of log(1+x) and further for cases where x tends to 0 ( because x = r/n and n tends to infinity) log(1+x) can be approximated to just x by ignoring the higher power terms in the taylor series.
– Aditya
Sep 2 at 8:06










2 Answers
2






active

oldest

votes

















up vote
3
down vote



accepted










HINT:



Write beginalignfrac1rlogleft(1+frac rnright)&=frac1rleft[frac rn-frac12left(frac rnright)^2+frac13left(frac rnright)^3-cdotsright]\&=frac1n-frac r2n^2+fracr^23n^3-cdotsendalign so beginalignsum_r=1^nfraclogleft(1+fracrnright)r&=1-fracsum r2n^2+fracsum r^23n^3-cdots\&=1-frac12int_0^1r,dr+frac13int_0^1r^2,dr-cdotsendalign as each term is essentially a Riemann sum.






share|cite|improve this answer


















  • 1




    Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
    – Batominovski
    Sep 2 at 8:09










  • @Batominovski Nice. I'll add this in.
    – TheSimpliFire
    Sep 2 at 8:10










  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:13











  • @tatan You can't take limits whilst the term is being multiplied by another in a summation.
    – TheSimpliFire
    Sep 2 at 8:15










  • @TheSimpliFire Okay thanks. But why can you please explain?
    – tatan
    Sep 2 at 8:15

















up vote
1
down vote













HINT



Following the previous suggestion by TheSimpliFire



$$sum_r=1^nfraclogleft(1+fracrnright)r=1-fracsum r2n^2+fracsum r^23n^3-cdotsto1-frac12cdot 2+frac13cdot 3ldots=$$$$=sum_k=0^infty frac(-1)^k(k+1)^2=sum_k=1^infty frac(-1)^k+1k^2=fracpi^212$$



indeed from the well known results



  • Different methods to compute $sumlimits_k=1^infty frac1k^2$ (Basel problem)

  • Sum of Reciprocals of Squares of Odd Integers

we have



$$sum_k=1^infty frac(-1)^k+1k^2=sum_k=1^infty frac1(2k-1)^2-sum_k=1^infty frac1(2k)^2=sum_k=1^infty frac1(2k-1)^2-frac14sum_k=1^infty frac1k^2=fracpi^28-fracpi^224=fracpi^212$$






share|cite|improve this answer






















  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:15










  • @tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
    – gimusi
    Sep 2 at 8:31










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
3
down vote



accepted










HINT:



Write beginalignfrac1rlogleft(1+frac rnright)&=frac1rleft[frac rn-frac12left(frac rnright)^2+frac13left(frac rnright)^3-cdotsright]\&=frac1n-frac r2n^2+fracr^23n^3-cdotsendalign so beginalignsum_r=1^nfraclogleft(1+fracrnright)r&=1-fracsum r2n^2+fracsum r^23n^3-cdots\&=1-frac12int_0^1r,dr+frac13int_0^1r^2,dr-cdotsendalign as each term is essentially a Riemann sum.






share|cite|improve this answer


















  • 1




    Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
    – Batominovski
    Sep 2 at 8:09










  • @Batominovski Nice. I'll add this in.
    – TheSimpliFire
    Sep 2 at 8:10










  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:13











  • @tatan You can't take limits whilst the term is being multiplied by another in a summation.
    – TheSimpliFire
    Sep 2 at 8:15










  • @TheSimpliFire Okay thanks. But why can you please explain?
    – tatan
    Sep 2 at 8:15














up vote
3
down vote



accepted










HINT:



Write beginalignfrac1rlogleft(1+frac rnright)&=frac1rleft[frac rn-frac12left(frac rnright)^2+frac13left(frac rnright)^3-cdotsright]\&=frac1n-frac r2n^2+fracr^23n^3-cdotsendalign so beginalignsum_r=1^nfraclogleft(1+fracrnright)r&=1-fracsum r2n^2+fracsum r^23n^3-cdots\&=1-frac12int_0^1r,dr+frac13int_0^1r^2,dr-cdotsendalign as each term is essentially a Riemann sum.






share|cite|improve this answer


















  • 1




    Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
    – Batominovski
    Sep 2 at 8:09










  • @Batominovski Nice. I'll add this in.
    – TheSimpliFire
    Sep 2 at 8:10










  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:13











  • @tatan You can't take limits whilst the term is being multiplied by another in a summation.
    – TheSimpliFire
    Sep 2 at 8:15










  • @TheSimpliFire Okay thanks. But why can you please explain?
    – tatan
    Sep 2 at 8:15












up vote
3
down vote



accepted







up vote
3
down vote



accepted






HINT:



Write beginalignfrac1rlogleft(1+frac rnright)&=frac1rleft[frac rn-frac12left(frac rnright)^2+frac13left(frac rnright)^3-cdotsright]\&=frac1n-frac r2n^2+fracr^23n^3-cdotsendalign so beginalignsum_r=1^nfraclogleft(1+fracrnright)r&=1-fracsum r2n^2+fracsum r^23n^3-cdots\&=1-frac12int_0^1r,dr+frac13int_0^1r^2,dr-cdotsendalign as each term is essentially a Riemann sum.






share|cite|improve this answer














HINT:



Write beginalignfrac1rlogleft(1+frac rnright)&=frac1rleft[frac rn-frac12left(frac rnright)^2+frac13left(frac rnright)^3-cdotsright]\&=frac1n-frac r2n^2+fracr^23n^3-cdotsendalign so beginalignsum_r=1^nfraclogleft(1+fracrnright)r&=1-fracsum r2n^2+fracsum r^23n^3-cdots\&=1-frac12int_0^1r,dr+frac13int_0^1r^2,dr-cdotsendalign as each term is essentially a Riemann sum.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Sep 2 at 8:11

























answered Sep 2 at 8:02









TheSimpliFire

10.8k62054




10.8k62054







  • 1




    Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
    – Batominovski
    Sep 2 at 8:09










  • @Batominovski Nice. I'll add this in.
    – TheSimpliFire
    Sep 2 at 8:10










  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:13











  • @tatan You can't take limits whilst the term is being multiplied by another in a summation.
    – TheSimpliFire
    Sep 2 at 8:15










  • @TheSimpliFire Okay thanks. But why can you please explain?
    – tatan
    Sep 2 at 8:15












  • 1




    Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
    – Batominovski
    Sep 2 at 8:09










  • @Batominovski Nice. I'll add this in.
    – TheSimpliFire
    Sep 2 at 8:10










  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:13











  • @tatan You can't take limits whilst the term is being multiplied by another in a summation.
    – TheSimpliFire
    Sep 2 at 8:15










  • @TheSimpliFire Okay thanks. But why can you please explain?
    – tatan
    Sep 2 at 8:15







1




1




Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
– Batominovski
Sep 2 at 8:09




Or treat each term $$frac(-1)^k(k+1),n^k+1,sum_r=1^n,r^k$$ as the Riemann sum for $$frac(-1)^k(k+1),int_0^1,x^k,textdx,.$$
– Batominovski
Sep 2 at 8:09












@Batominovski Nice. I'll add this in.
– TheSimpliFire
Sep 2 at 8:10




@Batominovski Nice. I'll add this in.
– TheSimpliFire
Sep 2 at 8:10












Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
– tatan
Sep 2 at 8:13





Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
– tatan
Sep 2 at 8:13













@tatan You can't take limits whilst the term is being multiplied by another in a summation.
– TheSimpliFire
Sep 2 at 8:15




@tatan You can't take limits whilst the term is being multiplied by another in a summation.
– TheSimpliFire
Sep 2 at 8:15












@TheSimpliFire Okay thanks. But why can you please explain?
– tatan
Sep 2 at 8:15




@TheSimpliFire Okay thanks. But why can you please explain?
– tatan
Sep 2 at 8:15










up vote
1
down vote













HINT



Following the previous suggestion by TheSimpliFire



$$sum_r=1^nfraclogleft(1+fracrnright)r=1-fracsum r2n^2+fracsum r^23n^3-cdotsto1-frac12cdot 2+frac13cdot 3ldots=$$$$=sum_k=0^infty frac(-1)^k(k+1)^2=sum_k=1^infty frac(-1)^k+1k^2=fracpi^212$$



indeed from the well known results



  • Different methods to compute $sumlimits_k=1^infty frac1k^2$ (Basel problem)

  • Sum of Reciprocals of Squares of Odd Integers

we have



$$sum_k=1^infty frac(-1)^k+1k^2=sum_k=1^infty frac1(2k-1)^2-sum_k=1^infty frac1(2k)^2=sum_k=1^infty frac1(2k-1)^2-frac14sum_k=1^infty frac1k^2=fracpi^28-fracpi^224=fracpi^212$$






share|cite|improve this answer






















  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:15










  • @tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
    – gimusi
    Sep 2 at 8:31














up vote
1
down vote













HINT



Following the previous suggestion by TheSimpliFire



$$sum_r=1^nfraclogleft(1+fracrnright)r=1-fracsum r2n^2+fracsum r^23n^3-cdotsto1-frac12cdot 2+frac13cdot 3ldots=$$$$=sum_k=0^infty frac(-1)^k(k+1)^2=sum_k=1^infty frac(-1)^k+1k^2=fracpi^212$$



indeed from the well known results



  • Different methods to compute $sumlimits_k=1^infty frac1k^2$ (Basel problem)

  • Sum of Reciprocals of Squares of Odd Integers

we have



$$sum_k=1^infty frac(-1)^k+1k^2=sum_k=1^infty frac1(2k-1)^2-sum_k=1^infty frac1(2k)^2=sum_k=1^infty frac1(2k-1)^2-frac14sum_k=1^infty frac1k^2=fracpi^28-fracpi^224=fracpi^212$$






share|cite|improve this answer






















  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:15










  • @tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
    – gimusi
    Sep 2 at 8:31












up vote
1
down vote










up vote
1
down vote









HINT



Following the previous suggestion by TheSimpliFire



$$sum_r=1^nfraclogleft(1+fracrnright)r=1-fracsum r2n^2+fracsum r^23n^3-cdotsto1-frac12cdot 2+frac13cdot 3ldots=$$$$=sum_k=0^infty frac(-1)^k(k+1)^2=sum_k=1^infty frac(-1)^k+1k^2=fracpi^212$$



indeed from the well known results



  • Different methods to compute $sumlimits_k=1^infty frac1k^2$ (Basel problem)

  • Sum of Reciprocals of Squares of Odd Integers

we have



$$sum_k=1^infty frac(-1)^k+1k^2=sum_k=1^infty frac1(2k-1)^2-sum_k=1^infty frac1(2k)^2=sum_k=1^infty frac1(2k-1)^2-frac14sum_k=1^infty frac1k^2=fracpi^28-fracpi^224=fracpi^212$$






share|cite|improve this answer














HINT



Following the previous suggestion by TheSimpliFire



$$sum_r=1^nfraclogleft(1+fracrnright)r=1-fracsum r2n^2+fracsum r^23n^3-cdotsto1-frac12cdot 2+frac13cdot 3ldots=$$$$=sum_k=0^infty frac(-1)^k(k+1)^2=sum_k=1^infty frac(-1)^k+1k^2=fracpi^212$$



indeed from the well known results



  • Different methods to compute $sumlimits_k=1^infty frac1k^2$ (Basel problem)

  • Sum of Reciprocals of Squares of Odd Integers

we have



$$sum_k=1^infty frac(-1)^k+1k^2=sum_k=1^infty frac1(2k-1)^2-sum_k=1^infty frac1(2k)^2=sum_k=1^infty frac1(2k-1)^2-frac14sum_k=1^infty frac1k^2=fracpi^28-fracpi^224=fracpi^212$$







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Sep 2 at 8:27

























answered Sep 2 at 8:13









gimusi

72.2k73888




72.2k73888











  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:15










  • @tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
    – gimusi
    Sep 2 at 8:31
















  • Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
    – tatan
    Sep 2 at 8:15










  • @tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
    – gimusi
    Sep 2 at 8:31















Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
– tatan
Sep 2 at 8:15




Can't this be done without using $log(1+x)$ expansion? I was trying $lim_ntoinftysum_r=1^nfraclog(1+frac rn)r=lim_xto0sum_r=1^nfraclog(1+x)xcdotfrac1n$ where $x=frac rn$... Now the log part goes to $1$ (by using standard limit formula). Can the answer be derived from here?
– tatan
Sep 2 at 8:15












@tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
– gimusi
Sep 2 at 8:31




@tatan No we can't since we need all the terms of the expansion and take 1 corresponds to take only one terms which leads to a wrong result.
– gimusi
Sep 2 at 8:31

















 

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