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Numerically integrate bivariate function

  1. Feb 12, 2016 #1
    What methods are available for integrating, e.g.
    [itex]
    \int^{\infty}_{0} f(x) dx \int^{x}_{0} g(x,y) dy
    [/itex]

    numerically without resorting to symbolic integration. Thanks
     
  2. jcsd
  3. Feb 12, 2016 #2

    RUber

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    Homework Helper

    Assuming that the integral exists and there is a sufficiently large x for which the function is approximately zero, then you could look at this as a bounded integral.
    ## \int_0^L\int_0^x f(x) g(x,y) dy dx ##
    Numerically, you would partition the interval from 0 to L with N+1 points ##\{x_i\}_{i=0}^N##, such that ##x_0 = 0 ## and ##x_N = L. ##
    Then you would have a region of integration that grows like half a square.

    If you apply a midpoint rule for a simple approximation, this might look like ( with ##dx_i = x_i - x_{i-1}##) :
    ## \int_0^L\int_0^x f(x) g(x,y) dy dx \approx \sum_{ i = 1 }^N dx_i f\left( \frac{x_i + x_{i-1}}{2}\right) \sum_{j=1}^i dx_j g\left(\frac{x_i + x_{i-1}}{2}, \frac{x_j + x_{j-1}}{2} \right)##

    Does that help?
     
  4. Feb 13, 2016 #3

    pasmith

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    I think I would first move to polar coordinates, so that [tex]I = \int_0^\infty \int_0^x f(x)g(x,y)\,dy\,dx =
    \int_0^\infty \int_0^{\pi/4} f(r\cos\theta) g(r\cos\theta, r\sin\theta) r\,d\theta\,dr.[/tex] Then I'd set [itex]r = \mathrm{arctanh}(s)[/itex] so that [tex]
    I = \int_0^1 \int_0^{\pi/4} f(\mathrm{arctanh}(s)\cos\theta) g(\mathrm{arctanh}(s)\cos\theta, \mathrm{arctanh}(s)\sin\theta) \frac{\mathrm{arctanh}(s)}{1 - s^2}\,d\theta\,ds.[/tex]
    Now I'm integrating over a finite rectangle, and I probably want to use an algorithm which doesn't require me to evaluate the integrand at [itex]s = 1[/itex].

    Alternatively, I could split the integral as [tex]
    \int_0^1 \int_0^{\pi/4} f(r\cos\theta) g(r\cos\theta, r\sin\theta) r\,d\theta\,dr + \int_1^\infty \int_0^{\pi/4} f(r\cos\theta) g(r\cos\theta, r\sin\theta) r\,d\theta\,dr[/tex]
    and in the second integral make the substitution [itex]z = 1/r[/itex], to obtain [tex]
    I = \int_0^1 \int_0^{\pi/4} f(r\cos\theta) g(r\cos\theta, r\sin\theta) r
    + f(r^{-1}\cos\theta)g(r^{-1}\cos\theta,r^{-1}\sin\theta)r^{-3}\,d\theta\,dr
    [/tex] and this time I probably want to use an algorithm which doesn't require me to evaluate the integrand at [itex]r = 0[/itex].
     
  5. Feb 13, 2016 #4
    Thanks, the change of variables is intuitive, but I don't know any algorithms?
    Also, in the last step in your alternative procedure shouldn't the second integral be over z not r?
     
    Last edited: Feb 13, 2016
  6. Feb 13, 2016 #5

    Ssnow

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    Gold Member

    For algorithms I suggest the Burden Faires, 8th edition, there is a section regarding the approximations of single and double integrals ...
     
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