# Integrate a Bessel Function K0(a*x) without the constant 'a' ?

Posted 6 days ago
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 Thanks for reading !I am having a problem with the numerical integration of the Bessel functiony = BesselK0(a*x)Since my constant is too large (a = 6800) I am getting the large arguments aproximation for the Bessel K0 function, making the integration difficult.My question is: Is there some mathematical method or manipulation to remove the constant 'a' from the integration likey = BesselK0(x)to make me integrate it like small arguments and then insert the constant 'a' in it somehow after ?Thank you very much !
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Posted 6 days ago
 Integrate[BesselK[0, a x], x] gives: 1/2 \[Pi] x (BesselK[0,a x] StruveL[-1,a x]+BesselK[1,a x] StruveL[0,a x]) Where you then can plug in the end-points…
Posted 6 days ago
 For large arguments, you might consider looking into the asymptotic expansion of the modified Bessel function (e.g. this one).
Posted 6 days ago
 Let me clarify my question:I am using the online paid wolfram alpha.Thank you two for the responses, I am trying to implement a fortran code here to Integrate this bessel function wich is a series of functions that multiply that bessel function, and the online wolfram cannot integrate analitcaly. The true form of the function is: NIntegrate[(Sqrt[(0. + 0.0078125*(1 - xi) + 0.0078125*xi)^2]*(-0.015625 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))* BesselK[0, 6283.185307179586* Sqrt[0. + (-0.015625 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))^2]])/ (0. + (-0.015625 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))^2), {xi, -1, 1}] The constant '6283.185307179586' is the constant 'a' that I said. If it is small, I can integrate it since it has a LOG singularity BY USING GAUSS LOG INTEGRATION. If the constant 'a' is large, I cant integrate it since it has finite part integrals of many orders ( large argument expansion). Is there any mathematical method ( change of variables ?, substitution ? ) that can make me remove this 'a' '6283.185307179586' from the integral ? or make the argument small ?
Posted 6 days ago
 func = Rationalize[(Sqrt[(0. + 0.0078125 (1 - xi) + 0.0078125 xi)^2] (-0.015625 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi)) BesselK[0, 6283.185307179586* Sqrt[0. + (-0.015625 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi))^2]])/(0. + (-0.015625 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi))^2), 0] // FullSimplify Integrate[func, {xi, -1, 1}](*Integral is divergent*) Integrate[BesselK[0, 2 Sqrt[(-1 + xi)^2]]/(-1 + xi), {xi, -1, 1}] (*Simplified version.Integral is divergent*) Removing this: '6283.185307179586' large number will not help you at all.
Posted 6 days ago
 Thank you very much for playing with the integral, yes this one is divergent !! But this is at one node of the problem, I am also dealing with integrals that are not divergent like this one: Integrate[(Sqrt[(0. + 0.0078125*(1 - xi) + 0.0078125*xi)^2]*(-0.0078125 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))* BesselK[0, 6283.185307179586* Sqrt[0. + (-0.0078125 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))^2]])/ (0. + (-0.0078125 + 0.0078125*(1 - xi)*(1 + xi) + 0.0078125*xi*(1 + xi))^2), {xi, -1, 1}] If the correct numerical integration is used, this integral is given = 0. So if the constant is used 6283.185307179586, one need to solve as a cauchy principal value integral, if the constant is small one could solve as a weakly singular integral. I would love to remove that constant from the Bessel Term and then add it somehow after the integration. But Thank you very much for your answer !
Posted 6 days ago
 func = Rationalize[(Sqrt[(0. + 0.0078125 (1 - xi) + 0.0078125 xi)^2] (-0.0078125 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi)) BesselK[ 0, 6283.185307179586* Sqrt[0. + (-0.0078125 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi))^2]])/(0. + (-0.0078125 + 0.0078125 (1 - xi) (1 + xi) + 0.0078125 xi (1 + xi))^2), 0] // FullSimplify; NIntegrate[func, {xi, -1, 0, 1}, Method -> PrincipalValue] (* 2.52435*10^-29 *) With the Cauchy principal value integral is finite and equal to Zero.
Posted 6 days ago
 This is a singular integral, maybe mathematica is having some problems integrating it ?
Posted 6 days ago
 It's seems a bug is in Integrate command and Yes you are right. Integrate[BesselK[0, (6632555543 Sqrt[xi^2])/135117312]/xi, {xi, -1, 1}, PrincipalValue -> True](* It should be Zero*) Indefine integral: Integrate[BesselK[0, Sqrt[x^2]]/x, x](*Cant find !*) but: $$\int \frac{K_0\left(\sqrt{x^2}\right)}{x} \, dx=-\frac{1}{4} G_{1,3}^{3,0}\left(\frac{x^2}{4}| \begin{array}{c} 1 \\ 0,0,0 \\ \end{array} \right)+C$$With Cauchy Principal Value: Limit[((-Inactive[MeijerG][{{}, {1}}, {{0, 0, 0}, {}}, x^2/4]/4 // Activate) /. x -> (-e)) - ((-Inactive[MeijerG][{{}, {1}}, {{0, 0, 0}, {}}, x^2/ 4]/4 // Activate) /. x -> -1) + (((-Inactive[MeijerG][{{}, {1}}, {{0, 0, 0}, {}}, x^2/ 4]/4 // Activate) /. x -> 1) - ((-Inactive[MeijerG][{{}, {1}}, {{0, 0, 0}, {}}, x^2/ 4]/4 // Activate) /. x -> e)), e -> 0, Direction -> -1] (* 0 *)