# Spherical Bessel Zeros¶

It may be useful to find out the zeros of the spherical Bessel functions, for instance, if you want to compute the eigenfrequencies of a spherical electromagnetic cavity (in this case, you'll need also the zeros of the derivative of (r*Jn(r))).

The problem is that you have to work out the ranges where you are supposed to find the zeros.

Happily, the range of a given zero of the n'th spherical Bessel functions can be computed from the zeros of the (n-1)'th spherical Bessel function.

Thus, the approach proposed here is recursive, knowing that the spherical Bessel function of order 0 is equal to sin(r)/r, whose zeros are well known.

This approach is obviously not efficient at all, but it works ;-).

A sample example is shown, for the 10 first zeros of the spherical Bessel function of order 5 (and the derivative of (r*J5(r))), using [:Cookbook/Matplotlib: matplotlib].

In [ ]:
#! /usr/bin/env python

### recursive method: computes zeros ranges of Jn(r,n) from zeros of Jn(r,n-1)
### (also for zeros of (rJn(r,n))')
### pros : you are certain to find the right zeros values;
### cons : all zeros of the n-1 previous Jn have to be computed;
### note : Jn(r,0) = sin(r)/r

from scipy import arange, pi, sqrt, zeros
from scipy.special import jv, jvp
from scipy.optimize import brentq
from sys import argv
from pylab import *

def Jn(r,n):
return (sqrt(pi/(2*r))*jv(n+0.5,r))
def Jn_zeros(n,nt):
zerosj = zeros((n+1, nt), dtype=Float32)
zerosj[0] = arange(1,nt+1)*pi
points = arange(1,nt+n+1)*pi
racines = zeros(nt+n, dtype=Float32)
for i in range(1,n+1):
for j in range(nt+n-i):
foo = brentq(Jn, points[j], points[j+1], (i,))
racines[j] = foo
points = racines
zerosj[i][:nt] = racines[:nt]
return (zerosj)

def rJnp(r,n):
return (0.5*sqrt(pi/(2*r))*jv(n+0.5,r) + sqrt(pi*r/2)*jvp(n+0.5,r))
def rJnp_zeros(n,nt):
zerosj = zeros((n+1, nt), dtype=Float32)
zerosj[0] = (2.*arange(1,nt+1)-1)*pi/2
points = (2.*arange(1,nt+n+1)-1)*pi/2
racines = zeros(nt+n, dtype=Float32)
for i in range(1,n+1):
for j in range(nt+n-i):
foo = brentq(rJnp, points[j], points[j+1], (i,))
racines[j] = foo
points = racines
zerosj[i][:nt] = racines[:nt]
return (zerosj)

n = int(argv[1])  # n'th spherical bessel function
nt = int(argv[2]) # number of zeros to be computed

dr = 0.01
eps = dr/1000

jnz = Jn_zeros(n,nt)[n]
r1 = arange(eps,jnz[len(jnz)-1],dr)
jnzp = rJnp_zeros(n,nt)[n]
r2 = arange(eps,jnzp[len(jnzp)-1],dr)

grid(True)
plot(r1,Jn(r1,n),'b', r2,rJnp(r2,n),'r')
title((str(nt)+' first zeros'))
legend((r'$j_{'+str(n)+'}(r)$', r'$(rj_{'+str(n)+'}(r))\'$'))
plot(jnz,zeros(len(jnz)),'bo', jnzp,zeros(len(jnzp)),'rd')
gca().xaxis.set_minor_locator(MultipleLocator(1))
# gca().xaxis.set_minor_formatter(FormatStrFormatter('%d'))
show()

In [ ]:
bessph_zeros_rec 5 10


Section author: FredericPetit, GaelVaroquaux

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