Updates:
        Labels: WrongResult Integration

Comment #1 on issue 1576 by asmeurer: Integral of strickly positive  
function is zero
http://code.google.com/p/sympy/issues/detail?id=1576

What do you have f defined as?  You can't have it defined as the default  
function from isympy, because I get:

$isympy -c python
Python 2.6.2 console for SymPy 0.6.5

These commands were executed:
>>> from __future__ import division
>>> from sympy import *
>>> x, y, z = symbols('xyz')
>>> k, m, n = symbols('kmn', integer=True)
>>> f, g, h = map(Function, 'fgh')

Documentation can be found at http://sympy.org/

>>> fn=Symbol('fn', positive=True)
>>> 4*pi**2*f**2*fn**4*(fn**2+9*f**2)/(fn**2+f**2)**3
Traceback (most recent call last):
   File "<console>", line 1, in <module>
TypeError: unsupported operand type(s) for ** or pow(): 'FunctionClass'  
and 'int'

By the way, according to Maple, the Maxima result is correct:

> assume(fn, positive);
> 4*pi**2*f**2*fn**4*(fn**2+9*f**2)/(fn**2+f**2)**3;
                               2  2   4 /  2      2\
                           4 pi  f  fn  \fn  + 9 f /
                           -------------------------
                                            3
                                  /  2    2\
                                  \fn  + f /
> int(4*pi**2*f**2*fn**4*(fn**2+9*f**2)/(fn**2+f**2)**3,  
> f=-infinity..infinity);
                                      2   3
                                 14 pi  fn  Pi


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