Large array returned from scipy integrate quad

Question

everyone. I was trying to compute an integral numerically using scipy.quad. However, it returns a really huge array with no other error. Can anyone see the problem? The code works very fine with simple integral like quad(lambda x:x**2,0,4) but for the code below try a=20,b=20,c=20,j=1 you get a really huge list of result with repeating numbers

def Lj(a,b,c,j):
    R1=a
    R2=b
    R3=c
    if j==1:
        Rj=a
    elif j==2:
        Rj=b
    elif j==3:
        Rj=c
    else:
        raise ValueError('j needs to be an integer between 1 and 3')

    result = (R1*R2*R3)*quad(lambda s: 1/((s+Rj**2)*sqrt(((s+R1**2)*(s+R2**2)*(s+R3**2))))/2, 0, inf)
    return result

enter image description here

Did you forget to post some code? Please post exact code which displays the problem and and exact traceback/output. — ev-br
– ev-br, Commented Mar 18, 2015 at 15:39

Warren Weckesser · Accepted Answer · 2015-03-18 16:59:18Z

3

quad returns a tuple containing the integral and an estimate of the error. You multiply this tuple by R1*R2*R3, which replicates the tuple R1*R2*R3 times.

To fix this, use just the first value returned by quad. That is, replace this:

result = (R1*R2*R3)*quad(lambda s: 1/((s+Rj**2)*sqrt(((s+R1**2)*(s+R2**2)*(s+R3**2))))/2, 0, inf)

with something like this:

intgrl, err = quad(lambda s: 1/((s+Rj**2)*sqrt(((s+R1**2)*(s+R2**2)*(s+R3**2))))/2, 0, inf)
result = (R1*R2*R3)*intgrl

edited Mar 18, 2015 at 16:59

answered Mar 18, 2015 at 16:45

Warren Weckesser

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