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Calculating spherical Bessel functions

  1. Jan 23, 2006 #1
    I'm not really sure which forum this is appropriate for, so I'll put it here and hope someone can help! I can't find the link to the LaTeX guide that used to be in General Physics (I think), so underscores will have to do for saying something is a subscript.

    I have to write a program to calculate the spherical Bessel function, j_l (actually, I need to calculate j_5(x) for x = 0.1, 1 and 10). We are given a recursion relation:

    j_(l - 1) = (j_l)*(2l + 1)/x - j_(l + 1)

    We're also told that j_0 = (sin x)/x and that we should pick values for "large" l, so we can define j_l = j_(l + 1) = 1. I've tried l = 100 and have written a program as follows (in C++):

    The user is prompted for a value of x and then the value of j_0 is calculated. I then declare and initialise 4 variables, one to hold each of j_l, j_(l + 1), j_(l - 1) and j_5 when it's found. The first two are given the value one (since we're given that) and the other two the value 0 (for the time being).

    Then, I have a for loop that decrements l each time through, going from 100 to 1 so that the value of j_(l - 1) can be calculated from j_(l + 1) and j_l (so when l = 100, the value of j_99 is calculated). The problem is, the values are too high:

    j_99 = 2009
    j_98 = 3.9979 x 10^6
    ...
    j_5 = 1.288 x 10^280! (Maple and MathCad tell me I should have a value of 2.60 x 10^-9).

    We're also told that we have to scale our values by j_0 (so have to compare the calculated value from the loop, with that calculated after the user entered the value for x). The one I get from the loop is of the order 1 x 10^289, whereas when I do the normal calculation, I get a value close to 1 (which is very close to what Maple tells me).

    I don't think the problem is with my code, because I get the same results when doing the calculation on paper (well, I only did the first few, I didn't go all the way to j_5).

    Where am I going wrong? Perhaps using a too high value of l? We weren't given a definition of "large", the word is in inverted commas on the sheet and that suggests that it probably shouldn't be too large. Even with using l = 10, I get j_5 = 1 x 10^11.

    Any help appreciated.
     
  2. jcsd
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