Can we refine this asymptotic for Laguerre polynomials?
I just found an interesting and useful limit for Laguerre polynomials:
$$lim_{n to infty} L_n left( frac{2r}{n+1/2} right)=J_0(2 sqrt{2r})$$
I'm using specifically this form of the argument because it's the one I'm working with in the application. Of course, we can set any fixed number instead of $1/2$ in the denominator.
I found this limit in a paper, which references G. Szego. Orthogonal Polynomials. Amer. Math. Soc. Colloq. Publ. 23, Amer. Math. Soc. Providence,
RI, 1975. Fourth Edition., , Theorem 8.1.3.
While the limit is useful for very large orders and smallish $r$, I would really like to know if there's a refinement that could be applied to derive an asymptotic expansion, which would depend on $n$.
Here's an illustration which shows that the limit is not that good for larger $r$ (though it does approximate the roots better than the magnitude):
Not sure how we could refine this asymptotic or how the original limit was derived (as I don't have the linked book).
One way is considering the differential equations for both functions.
There's also an interesting result from Gradshteyn-Ryzhik:
$$L_n(z)= frac{2}{n!} e^z int_0^infty e^{-t^2} t^{2n+1} J_0(2t sqrt{z}) dt$$
Which may or may not be related to the limit above.
limits asymptotics approximation bessel-functions orthogonal-polynomials
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I just found an interesting and useful limit for Laguerre polynomials:
$$lim_{n to infty} L_n left( frac{2r}{n+1/2} right)=J_0(2 sqrt{2r})$$
I'm using specifically this form of the argument because it's the one I'm working with in the application. Of course, we can set any fixed number instead of $1/2$ in the denominator.
I found this limit in a paper, which references G. Szego. Orthogonal Polynomials. Amer. Math. Soc. Colloq. Publ. 23, Amer. Math. Soc. Providence,
RI, 1975. Fourth Edition., , Theorem 8.1.3.
While the limit is useful for very large orders and smallish $r$, I would really like to know if there's a refinement that could be applied to derive an asymptotic expansion, which would depend on $n$.
Here's an illustration which shows that the limit is not that good for larger $r$ (though it does approximate the roots better than the magnitude):
Not sure how we could refine this asymptotic or how the original limit was derived (as I don't have the linked book).
One way is considering the differential equations for both functions.
There's also an interesting result from Gradshteyn-Ryzhik:
$$L_n(z)= frac{2}{n!} e^z int_0^infty e^{-t^2} t^{2n+1} J_0(2t sqrt{z}) dt$$
Which may or may not be related to the limit above.
limits asymptotics approximation bessel-functions orthogonal-polynomials
add a comment |
I just found an interesting and useful limit for Laguerre polynomials:
$$lim_{n to infty} L_n left( frac{2r}{n+1/2} right)=J_0(2 sqrt{2r})$$
I'm using specifically this form of the argument because it's the one I'm working with in the application. Of course, we can set any fixed number instead of $1/2$ in the denominator.
I found this limit in a paper, which references G. Szego. Orthogonal Polynomials. Amer. Math. Soc. Colloq. Publ. 23, Amer. Math. Soc. Providence,
RI, 1975. Fourth Edition., , Theorem 8.1.3.
While the limit is useful for very large orders and smallish $r$, I would really like to know if there's a refinement that could be applied to derive an asymptotic expansion, which would depend on $n$.
Here's an illustration which shows that the limit is not that good for larger $r$ (though it does approximate the roots better than the magnitude):
Not sure how we could refine this asymptotic or how the original limit was derived (as I don't have the linked book).
One way is considering the differential equations for both functions.
There's also an interesting result from Gradshteyn-Ryzhik:
$$L_n(z)= frac{2}{n!} e^z int_0^infty e^{-t^2} t^{2n+1} J_0(2t sqrt{z}) dt$$
Which may or may not be related to the limit above.
limits asymptotics approximation bessel-functions orthogonal-polynomials
I just found an interesting and useful limit for Laguerre polynomials:
$$lim_{n to infty} L_n left( frac{2r}{n+1/2} right)=J_0(2 sqrt{2r})$$
I'm using specifically this form of the argument because it's the one I'm working with in the application. Of course, we can set any fixed number instead of $1/2$ in the denominator.
I found this limit in a paper, which references G. Szego. Orthogonal Polynomials. Amer. Math. Soc. Colloq. Publ. 23, Amer. Math. Soc. Providence,
RI, 1975. Fourth Edition., , Theorem 8.1.3.
While the limit is useful for very large orders and smallish $r$, I would really like to know if there's a refinement that could be applied to derive an asymptotic expansion, which would depend on $n$.
Here's an illustration which shows that the limit is not that good for larger $r$ (though it does approximate the roots better than the magnitude):
Not sure how we could refine this asymptotic or how the original limit was derived (as I don't have the linked book).
One way is considering the differential equations for both functions.
There's also an interesting result from Gradshteyn-Ryzhik:
$$L_n(z)= frac{2}{n!} e^z int_0^infty e^{-t^2} t^{2n+1} J_0(2t sqrt{z}) dt$$
Which may or may not be related to the limit above.
limits asymptotics approximation bessel-functions orthogonal-polynomials
limits asymptotics approximation bessel-functions orthogonal-polynomials
asked Nov 26 at 23:05
Yuriy S
15.7k433117
15.7k433117
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From the given representation, we can express
begin{equation}
L_n(frac{2r}{n+1/2})= frac{2}{n!} e^{frac{2r}{n+1/2}} int_0^infty e^{-t^2} t^{2n+1} J_0left(2t sqrt{frac{2r}{n+1/2}}right) ,dt
end{equation}
changing $ t=sqrt{uleft( n+1/2 right)}$,
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-uleft( n+1/2 right)}u^{n+1/2}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
or
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-left( n+1/2 right)left( u-ln u right)}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
The argument of the exponential is minimum at $u=1$. We can use the Laplace method to derive an asymptotic approximation for the integral. Near $u=1$,
$u-ln(u)sim 1+(u-1)^2/2$ and $u^{-1/2}J_0left( 2sqrt{2ru}right)sim J_0left( 2sqrt{2r}right)$, then
begin{equation}
L_n(frac{2r}{n+1/2})sim sqrt{2pi}frac{(n+1/2)^{n+1/2}}{n!} e^{frac{2r}{n+1/2}}
e^{-(n+1/2)}J_0left( 2sqrt{2r}right)
end{equation}
Now, plugging the Stirling approximation for $n!$ and an expansion for the exponential term, we obtain the expected result $ L_n(frac{2r}{n+1/2})sim J_0left( 2sqrt{2r}right)$. To improve the approximation, the above formula and/or higher orders in the Laplace expansion can be used.
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
add a comment |
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1 Answer
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1 Answer
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From the given representation, we can express
begin{equation}
L_n(frac{2r}{n+1/2})= frac{2}{n!} e^{frac{2r}{n+1/2}} int_0^infty e^{-t^2} t^{2n+1} J_0left(2t sqrt{frac{2r}{n+1/2}}right) ,dt
end{equation}
changing $ t=sqrt{uleft( n+1/2 right)}$,
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-uleft( n+1/2 right)}u^{n+1/2}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
or
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-left( n+1/2 right)left( u-ln u right)}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
The argument of the exponential is minimum at $u=1$. We can use the Laplace method to derive an asymptotic approximation for the integral. Near $u=1$,
$u-ln(u)sim 1+(u-1)^2/2$ and $u^{-1/2}J_0left( 2sqrt{2ru}right)sim J_0left( 2sqrt{2r}right)$, then
begin{equation}
L_n(frac{2r}{n+1/2})sim sqrt{2pi}frac{(n+1/2)^{n+1/2}}{n!} e^{frac{2r}{n+1/2}}
e^{-(n+1/2)}J_0left( 2sqrt{2r}right)
end{equation}
Now, plugging the Stirling approximation for $n!$ and an expansion for the exponential term, we obtain the expected result $ L_n(frac{2r}{n+1/2})sim J_0left( 2sqrt{2r}right)$. To improve the approximation, the above formula and/or higher orders in the Laplace expansion can be used.
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
add a comment |
From the given representation, we can express
begin{equation}
L_n(frac{2r}{n+1/2})= frac{2}{n!} e^{frac{2r}{n+1/2}} int_0^infty e^{-t^2} t^{2n+1} J_0left(2t sqrt{frac{2r}{n+1/2}}right) ,dt
end{equation}
changing $ t=sqrt{uleft( n+1/2 right)}$,
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-uleft( n+1/2 right)}u^{n+1/2}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
or
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-left( n+1/2 right)left( u-ln u right)}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
The argument of the exponential is minimum at $u=1$. We can use the Laplace method to derive an asymptotic approximation for the integral. Near $u=1$,
$u-ln(u)sim 1+(u-1)^2/2$ and $u^{-1/2}J_0left( 2sqrt{2ru}right)sim J_0left( 2sqrt{2r}right)$, then
begin{equation}
L_n(frac{2r}{n+1/2})sim sqrt{2pi}frac{(n+1/2)^{n+1/2}}{n!} e^{frac{2r}{n+1/2}}
e^{-(n+1/2)}J_0left( 2sqrt{2r}right)
end{equation}
Now, plugging the Stirling approximation for $n!$ and an expansion for the exponential term, we obtain the expected result $ L_n(frac{2r}{n+1/2})sim J_0left( 2sqrt{2r}right)$. To improve the approximation, the above formula and/or higher orders in the Laplace expansion can be used.
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
add a comment |
From the given representation, we can express
begin{equation}
L_n(frac{2r}{n+1/2})= frac{2}{n!} e^{frac{2r}{n+1/2}} int_0^infty e^{-t^2} t^{2n+1} J_0left(2t sqrt{frac{2r}{n+1/2}}right) ,dt
end{equation}
changing $ t=sqrt{uleft( n+1/2 right)}$,
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-uleft( n+1/2 right)}u^{n+1/2}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
or
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-left( n+1/2 right)left( u-ln u right)}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
The argument of the exponential is minimum at $u=1$. We can use the Laplace method to derive an asymptotic approximation for the integral. Near $u=1$,
$u-ln(u)sim 1+(u-1)^2/2$ and $u^{-1/2}J_0left( 2sqrt{2ru}right)sim J_0left( 2sqrt{2r}right)$, then
begin{equation}
L_n(frac{2r}{n+1/2})sim sqrt{2pi}frac{(n+1/2)^{n+1/2}}{n!} e^{frac{2r}{n+1/2}}
e^{-(n+1/2)}J_0left( 2sqrt{2r}right)
end{equation}
Now, plugging the Stirling approximation for $n!$ and an expansion for the exponential term, we obtain the expected result $ L_n(frac{2r}{n+1/2})sim J_0left( 2sqrt{2r}right)$. To improve the approximation, the above formula and/or higher orders in the Laplace expansion can be used.
From the given representation, we can express
begin{equation}
L_n(frac{2r}{n+1/2})= frac{2}{n!} e^{frac{2r}{n+1/2}} int_0^infty e^{-t^2} t^{2n+1} J_0left(2t sqrt{frac{2r}{n+1/2}}right) ,dt
end{equation}
changing $ t=sqrt{uleft( n+1/2 right)}$,
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-uleft( n+1/2 right)}u^{n+1/2}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
or
begin{equation}
L_n(frac{2r}{n+1/2})= frac{(n+1/2)^{n+1}}{n!} e^{frac{2r}{n+1/2}}int_0^infty e^{-left( n+1/2 right)left( u-ln u right)}J_0left( 2sqrt{2ru} right)frac{du}{sqrt{u}}
end{equation}
The argument of the exponential is minimum at $u=1$. We can use the Laplace method to derive an asymptotic approximation for the integral. Near $u=1$,
$u-ln(u)sim 1+(u-1)^2/2$ and $u^{-1/2}J_0left( 2sqrt{2ru}right)sim J_0left( 2sqrt{2r}right)$, then
begin{equation}
L_n(frac{2r}{n+1/2})sim sqrt{2pi}frac{(n+1/2)^{n+1/2}}{n!} e^{frac{2r}{n+1/2}}
e^{-(n+1/2)}J_0left( 2sqrt{2r}right)
end{equation}
Now, plugging the Stirling approximation for $n!$ and an expansion for the exponential term, we obtain the expected result $ L_n(frac{2r}{n+1/2})sim J_0left( 2sqrt{2r}right)$. To improve the approximation, the above formula and/or higher orders in the Laplace expansion can be used.
edited Nov 29 at 23:07
answered Nov 29 at 22:47
Paul Enta
4,11811029
4,11811029
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
add a comment |
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
Thank you! I will wait a little while before accepting the answer, but this is exactly what I wanted
– Yuriy S
Nov 30 at 1:02
add a comment |
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