import math


def frf(system):
    """Return the Frequency Response Function of the system.
    
    Returns a function which takes a frequency as an argument.
    This function when evaluated at any frequency returns the complex
    frequency response at that frequency.
    
    Example: frf(system)(10) returns the complex FRF at 10 Hz
    """
    def internal(f):
        c1,c2,r1,r2 = system
        w = 2 * math.pi * f
        num = 1 / (c1*c2*r1*r2)
        den = 1 / (c1*c2*r1*r2) + (r1+r2)/(c1*r1*r2) * 1j*w - w**2
        return num/den
    
    return internal