Integral involving hypergeometric function












2












$begingroup$


I've worked out the projection of a spherically symmetric power law volume density profile $rho(r)=br^a$, i.e. its surface density $sigma(R)$, and am now trying to integrate this in a series of annular bins to get the total mass in each, i.e. $int_A^Bsigma(R)R,{rm d}R$. (Actually the profile is a continuous series of power laws, but that's more of a footnote.)



I've worked just about everything out, but am stuck on an integral of the form:



$$I(r,R,a)=intleft(left(frac{r}{R}right)^2-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};-left(left(frac{r}{R}right)^2-1right)right){rm d}R$$



${}_2{rm F}_1$ is the hypergeometric function, as defined here. $a$ and $r$ are constants, in this context.



Given that the solution of this integral will be used to calculate the elements of a big-ish matrix and I have some iteration in mind, integrating numerically is... inconvenient, but I'm stumped on finding an analytic solution. Is there any hope here, or should I start thinking about efficient numerical solutions?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Try expanding the Hypergeometric then integrating term by term
    $endgroup$
    – clathratus
    Jan 10 at 19:34










  • $begingroup$
    @Kyle thank you very much for your generous bounty!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:07










  • $begingroup$
    @PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
    $endgroup$
    – Kyle
    Jan 22 at 9:20










  • $begingroup$
    I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:59
















2












$begingroup$


I've worked out the projection of a spherically symmetric power law volume density profile $rho(r)=br^a$, i.e. its surface density $sigma(R)$, and am now trying to integrate this in a series of annular bins to get the total mass in each, i.e. $int_A^Bsigma(R)R,{rm d}R$. (Actually the profile is a continuous series of power laws, but that's more of a footnote.)



I've worked just about everything out, but am stuck on an integral of the form:



$$I(r,R,a)=intleft(left(frac{r}{R}right)^2-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};-left(left(frac{r}{R}right)^2-1right)right){rm d}R$$



${}_2{rm F}_1$ is the hypergeometric function, as defined here. $a$ and $r$ are constants, in this context.



Given that the solution of this integral will be used to calculate the elements of a big-ish matrix and I have some iteration in mind, integrating numerically is... inconvenient, but I'm stumped on finding an analytic solution. Is there any hope here, or should I start thinking about efficient numerical solutions?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Try expanding the Hypergeometric then integrating term by term
    $endgroup$
    – clathratus
    Jan 10 at 19:34










  • $begingroup$
    @Kyle thank you very much for your generous bounty!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:07










  • $begingroup$
    @PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
    $endgroup$
    – Kyle
    Jan 22 at 9:20










  • $begingroup$
    I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:59














2












2








2





$begingroup$


I've worked out the projection of a spherically symmetric power law volume density profile $rho(r)=br^a$, i.e. its surface density $sigma(R)$, and am now trying to integrate this in a series of annular bins to get the total mass in each, i.e. $int_A^Bsigma(R)R,{rm d}R$. (Actually the profile is a continuous series of power laws, but that's more of a footnote.)



I've worked just about everything out, but am stuck on an integral of the form:



$$I(r,R,a)=intleft(left(frac{r}{R}right)^2-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};-left(left(frac{r}{R}right)^2-1right)right){rm d}R$$



${}_2{rm F}_1$ is the hypergeometric function, as defined here. $a$ and $r$ are constants, in this context.



Given that the solution of this integral will be used to calculate the elements of a big-ish matrix and I have some iteration in mind, integrating numerically is... inconvenient, but I'm stumped on finding an analytic solution. Is there any hope here, or should I start thinking about efficient numerical solutions?










share|cite|improve this question









$endgroup$




I've worked out the projection of a spherically symmetric power law volume density profile $rho(r)=br^a$, i.e. its surface density $sigma(R)$, and am now trying to integrate this in a series of annular bins to get the total mass in each, i.e. $int_A^Bsigma(R)R,{rm d}R$. (Actually the profile is a continuous series of power laws, but that's more of a footnote.)



I've worked just about everything out, but am stuck on an integral of the form:



$$I(r,R,a)=intleft(left(frac{r}{R}right)^2-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};-left(left(frac{r}{R}right)^2-1right)right){rm d}R$$



${}_2{rm F}_1$ is the hypergeometric function, as defined here. $a$ and $r$ are constants, in this context.



Given that the solution of this integral will be used to calculate the elements of a big-ish matrix and I have some iteration in mind, integrating numerically is... inconvenient, but I'm stumped on finding an analytic solution. Is there any hope here, or should I start thinking about efficient numerical solutions?







calculus integration indefinite-integrals hypergeometric-function






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share|cite|improve this question











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asked Jan 10 at 15:43









KyleKyle

18328




18328












  • $begingroup$
    Try expanding the Hypergeometric then integrating term by term
    $endgroup$
    – clathratus
    Jan 10 at 19:34










  • $begingroup$
    @Kyle thank you very much for your generous bounty!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:07










  • $begingroup$
    @PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
    $endgroup$
    – Kyle
    Jan 22 at 9:20










  • $begingroup$
    I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:59


















  • $begingroup$
    Try expanding the Hypergeometric then integrating term by term
    $endgroup$
    – clathratus
    Jan 10 at 19:34










  • $begingroup$
    @Kyle thank you very much for your generous bounty!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:07










  • $begingroup$
    @PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
    $endgroup$
    – Kyle
    Jan 22 at 9:20










  • $begingroup$
    I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
    $endgroup$
    – Paul Enta
    Jan 22 at 9:59
















$begingroup$
Try expanding the Hypergeometric then integrating term by term
$endgroup$
– clathratus
Jan 10 at 19:34




$begingroup$
Try expanding the Hypergeometric then integrating term by term
$endgroup$
– clathratus
Jan 10 at 19:34












$begingroup$
@Kyle thank you very much for your generous bounty!
$endgroup$
– Paul Enta
Jan 22 at 9:07




$begingroup$
@Kyle thank you very much for your generous bounty!
$endgroup$
– Paul Enta
Jan 22 at 9:07












$begingroup$
@PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
$endgroup$
– Kyle
Jan 22 at 9:20




$begingroup$
@PaulEnta thanks for the answer! The solution of the integral goes into a computation for a measurement in astrophysics/cosmology I'm collaborating on at the moment, which we should start writing up for a journal soon. If you would like, I'd be happy to add an acknowledgement of your contribution along the lines of "The authors thank the math.stackexchange.com user Paul Enta for providing an analytic expression for eq X", or similar. There is usually a short section with such notes anyway. You may reach me by email at either of the addresses listed here.
$endgroup$
– Kyle
Jan 22 at 9:20












$begingroup$
I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
$endgroup$
– Paul Enta
Jan 22 at 9:59




$begingroup$
I am happy to see that this answer can be useful to you. Thank you for proposing to acknowledge this contribution in your future paper. I think that it can be a good advertisment for MSE in your community. I wish you the best for this article and for the next ones!
$endgroup$
– Paul Enta
Jan 22 at 9:59










1 Answer
1






active

oldest

votes


















5





+50







$begingroup$

In the following, we assume $R>r$.
begin{equation}
I(r,R,a)=intleft(frac{r^2}{R^2}-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};1-frac{r^2}{R^2}right){rm d}R
end{equation}

changing $u=1-r^2/R^2$, we obtain
begin{equation}
I(r,R,a)=ifrac{r^{a+3}}{2}int u^{frac12}left( 1-u right)^{-frac{a}{2}-frac{5}{2}}{}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};uright){rm d}u
end{equation}

where the principal determination for the square root was taken. Using the indefinite integration formula:
begin{equation}
int z^{c-1 }(1 - z)^{b - c-1}{}_2{rm F}_1left( a,b;c;z right)= z^c(1 - z)^{b - c} frac{Gamma(c)}{Gamma(c+1)} {}_2{rm F}_1left( 1+a,b;1+c;z right)
end{equation}

with $a=1/2,b=-a/2,c=3/2$, we find
begin{equation}
I(r,R,a)=-frac{1}{3}R^{a+3}left( frac{r^2}{R^2}-1 right)^{3/2} {}_2{rm F}_1left(frac{3}{2},-frac{a}{2};frac{5}{2};1-frac{r^2}{R^2}right)
end{equation}

which seems to be numerically correct.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
    $endgroup$
    – Kyle
    Jan 11 at 9:29






  • 1




    $begingroup$
    There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
    $endgroup$
    – Paul Enta
    Jan 11 at 9:33












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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









5





+50







$begingroup$

In the following, we assume $R>r$.
begin{equation}
I(r,R,a)=intleft(frac{r^2}{R^2}-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};1-frac{r^2}{R^2}right){rm d}R
end{equation}

changing $u=1-r^2/R^2$, we obtain
begin{equation}
I(r,R,a)=ifrac{r^{a+3}}{2}int u^{frac12}left( 1-u right)^{-frac{a}{2}-frac{5}{2}}{}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};uright){rm d}u
end{equation}

where the principal determination for the square root was taken. Using the indefinite integration formula:
begin{equation}
int z^{c-1 }(1 - z)^{b - c-1}{}_2{rm F}_1left( a,b;c;z right)= z^c(1 - z)^{b - c} frac{Gamma(c)}{Gamma(c+1)} {}_2{rm F}_1left( 1+a,b;1+c;z right)
end{equation}

with $a=1/2,b=-a/2,c=3/2$, we find
begin{equation}
I(r,R,a)=-frac{1}{3}R^{a+3}left( frac{r^2}{R^2}-1 right)^{3/2} {}_2{rm F}_1left(frac{3}{2},-frac{a}{2};frac{5}{2};1-frac{r^2}{R^2}right)
end{equation}

which seems to be numerically correct.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
    $endgroup$
    – Kyle
    Jan 11 at 9:29






  • 1




    $begingroup$
    There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
    $endgroup$
    – Paul Enta
    Jan 11 at 9:33
















5





+50







$begingroup$

In the following, we assume $R>r$.
begin{equation}
I(r,R,a)=intleft(frac{r^2}{R^2}-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};1-frac{r^2}{R^2}right){rm d}R
end{equation}

changing $u=1-r^2/R^2$, we obtain
begin{equation}
I(r,R,a)=ifrac{r^{a+3}}{2}int u^{frac12}left( 1-u right)^{-frac{a}{2}-frac{5}{2}}{}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};uright){rm d}u
end{equation}

where the principal determination for the square root was taken. Using the indefinite integration formula:
begin{equation}
int z^{c-1 }(1 - z)^{b - c-1}{}_2{rm F}_1left( a,b;c;z right)= z^c(1 - z)^{b - c} frac{Gamma(c)}{Gamma(c+1)} {}_2{rm F}_1left( 1+a,b;1+c;z right)
end{equation}

with $a=1/2,b=-a/2,c=3/2$, we find
begin{equation}
I(r,R,a)=-frac{1}{3}R^{a+3}left( frac{r^2}{R^2}-1 right)^{3/2} {}_2{rm F}_1left(frac{3}{2},-frac{a}{2};frac{5}{2};1-frac{r^2}{R^2}right)
end{equation}

which seems to be numerically correct.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
    $endgroup$
    – Kyle
    Jan 11 at 9:29






  • 1




    $begingroup$
    There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
    $endgroup$
    – Paul Enta
    Jan 11 at 9:33














5





+50







5





+50



5




+50



$begingroup$

In the following, we assume $R>r$.
begin{equation}
I(r,R,a)=intleft(frac{r^2}{R^2}-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};1-frac{r^2}{R^2}right){rm d}R
end{equation}

changing $u=1-r^2/R^2$, we obtain
begin{equation}
I(r,R,a)=ifrac{r^{a+3}}{2}int u^{frac12}left( 1-u right)^{-frac{a}{2}-frac{5}{2}}{}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};uright){rm d}u
end{equation}

where the principal determination for the square root was taken. Using the indefinite integration formula:
begin{equation}
int z^{c-1 }(1 - z)^{b - c-1}{}_2{rm F}_1left( a,b;c;z right)= z^c(1 - z)^{b - c} frac{Gamma(c)}{Gamma(c+1)} {}_2{rm F}_1left( 1+a,b;1+c;z right)
end{equation}

with $a=1/2,b=-a/2,c=3/2$, we find
begin{equation}
I(r,R,a)=-frac{1}{3}R^{a+3}left( frac{r^2}{R^2}-1 right)^{3/2} {}_2{rm F}_1left(frac{3}{2},-frac{a}{2};frac{5}{2};1-frac{r^2}{R^2}right)
end{equation}

which seems to be numerically correct.






share|cite|improve this answer









$endgroup$



In the following, we assume $R>r$.
begin{equation}
I(r,R,a)=intleft(frac{r^2}{R^2}-1right)^{frac{1}{2}}R^{a+2} {}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};1-frac{r^2}{R^2}right){rm d}R
end{equation}

changing $u=1-r^2/R^2$, we obtain
begin{equation}
I(r,R,a)=ifrac{r^{a+3}}{2}int u^{frac12}left( 1-u right)^{-frac{a}{2}-frac{5}{2}}{}_2{rm F}_1left(frac{1}{2},-frac{a}{2};frac{3}{2};uright){rm d}u
end{equation}

where the principal determination for the square root was taken. Using the indefinite integration formula:
begin{equation}
int z^{c-1 }(1 - z)^{b - c-1}{}_2{rm F}_1left( a,b;c;z right)= z^c(1 - z)^{b - c} frac{Gamma(c)}{Gamma(c+1)} {}_2{rm F}_1left( 1+a,b;1+c;z right)
end{equation}

with $a=1/2,b=-a/2,c=3/2$, we find
begin{equation}
I(r,R,a)=-frac{1}{3}R^{a+3}left( frac{r^2}{R^2}-1 right)^{3/2} {}_2{rm F}_1left(frac{3}{2},-frac{a}{2};frac{5}{2};1-frac{r^2}{R^2}right)
end{equation}

which seems to be numerically correct.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 10 at 21:25









Paul EntaPaul Enta

5,45611435




5,45611435












  • $begingroup$
    Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
    $endgroup$
    – Kyle
    Jan 11 at 9:29






  • 1




    $begingroup$
    There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
    $endgroup$
    – Paul Enta
    Jan 11 at 9:33


















  • $begingroup$
    Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
    $endgroup$
    – Kyle
    Jan 11 at 9:29






  • 1




    $begingroup$
    There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
    $endgroup$
    – Paul Enta
    Jan 11 at 9:33
















$begingroup$
Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
$endgroup$
– Kyle
Jan 11 at 9:29




$begingroup$
Checks out, many thanks! Out of curiosity, and for future use, what led you to that integral identity for the hypergeometric function?
$endgroup$
– Kyle
Jan 11 at 9:29




1




1




$begingroup$
There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
$endgroup$
– Paul Enta
Jan 11 at 9:33




$begingroup$
There are not so many indefinite integrals for the hypergeometric functions and yours seemed to be "well conditioned".
$endgroup$
– Paul Enta
Jan 11 at 9:33


















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