def create_tf(b, a):
    """ 
    Creates a transfer function given vectors of values for the numerator, b[i], and 
    denominator, a[i], 

             b[0]*s**n + b[1]*s**(n-1) + ... + b[n]
      f(s) = --------------------------------------
             a[0]*s**m + a[1]*s**(m-1) + ... + a[m]

    Arguments:
      b = transfer function numerator coefficients
      a = transfer function denominator coefficients

    Return:
     tf = transfer function

    """
    def tf(s):
        num = 0.0*s
        den = 0.0*s
        for i in range(b.shape[0]):
            num += b[i]*s**(b.shape[0]-i-1)
        for i in range(a.shape[0]):
            den += a[i]*s**(a.shape[0]-i-1)
        return num/den
    return tf