# Numerical integration problem: IntegrationWarning The integral is probably divergent, or slowly convergent

I am trying to get the numerical integration of a function using scipy's integrate.quad as follows.

$$\begin{equation} G (\alpha) = \frac{4\alpha}{\pi}\int_0^{\infty} x e^{-\alpha x^2} {(\pi/2}+\mathrm{tan}^{-1}[Y_0(x)/J_0 (x)]) \mathrm{d}x \end{equation}$$

import numpy as np
from scipy import integrate
from scipy.special import k0,j0,y0,k1
def G(alpha=743711.5,T=5.5e-5,sw=10.65):
pi = 3.14
fun = lambda x: x*np.exp(-1*alpha*x**2)*(pi/2+np.arctan(y0(x)/j0(x)))
return val,err
val,err = G()
print (val,err)


However, I get "The integral is probably divergent, or slowly convergent." I have tried to set a very large limit, such as limit = 10000000. However, the same warning is. Does anyone know how to solve the problem?

• Should $x$ be $u$ in that arctan function? Oct 9 '20 at 18:51
• Please make sure the question is correctly formulated. Do you integrate over $u$ or over $x$? Oct 10 '20 at 5:45
• Thank you. After I read the cited reference (Smith, 1937), I have found there are some errors in the above pasted source paper, and I have assured that the intergration is over x. I have corrected the above equation and code. However, the same warning is there. Oct 10 '20 at 14:09
• Have you tried to split the integral at the sum? I feel like the first summand could have an analytical solution, simplyfing the task and numerical effort Oct 15 '20 at 14:35

The problem is probably that the value of $$\alpha$$ is large compared to the values of $$x^2$$ that the numerical integrator is probably choosing. For any value $$x \gg 1/\sqrt{\alpha}$$ the integrand will be close to zero (for large enough values it will be exactly zero in floating point). The integrand is also zero at $$x = 0$$. So there is no point in setting the limit to a large value. On the contrary, try setting it to something like $$10/\sqrt{\alpha}, 20/\sqrt{\alpha}, ...$$ and check for convergence of the results.
Perhaps best would be to define a new variable $$u = x/\sqrt{\alpha}$$ and integrate with respect to $$u$$. Then the numerical integrator should probably work.