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Strange problem with double integrals

asked 2013-03-05 06:05:09 -0600

ebs gravatar image

updated 2013-03-05 06:08:47 -0600

I need to calculate a double of integral a large expression numerically. But, I found a strange error. The code:

def doubleIntegral():
    def f2(theta):
        def f1(phi):
            return theta*phi/pi
        return numerical_integral(f1,0,2*pi)[0]
    return numerical_integral(f2,0,pi)

works. but

a = theta*phi/pi  # Suppose this is a huge expression that came out of previous calculations

def doubleIntegral():
    def f2(theta):
        def f1(phi):
            return a
        return numerical_integral(f1,0,2*pi)[0]
    return numerical_integral(f2,0,pi)

doesn't work

unable to simplify to float approximation
unable to simplify to float approximation
.
.
.

What is going on? And how can I fix this?

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Comments

Try doing an internet search for using lambda functions for this. http://ask.sagemath.org/question/68/double-integral might be helpful along these lines...

kcrisman gravatar imagekcrisman ( 2013-03-05 06:30:15 -0600 )edit

my function doubleIntegral does the same thing as what that nestled numerical_integral with lambda functions (from that question) does. It has the exact same problem. Trying the internet search...

ebs gravatar imageebs ( 2013-03-05 06:41:30 -0600 )edit

1 answer

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answered 2013-03-05 07:38:26 -0600

ppurka gravatar image

In the second example function, you are returning a from the innermost loop, but a is a symbolic expression. In the first example, what you return from the innermost loop is just a number evaluated at theta and phi. What happens if you instead use return a.subs(theta=theta, phi=phi) in the second example?

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Oh! I get it. I also found dblquad in scipy.integrate (instead of this function) which seems to be faster. ty :)

ebs gravatar imageebs ( 2013-03-05 19:05:49 -0600 )edit

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Asked: 2013-03-05 06:05:09 -0600

Seen: 106 times

Last updated: Mar 05 '13