def intensity_parallel_from_monomer_n(M, A_1, A_2, b):
    """
    Calculate parallel intesity nominator from monomer fraction, 
    monomer and dimer anisotropy, brightness relation and monomer brightness.
    """
    return (2)*(((A_1+0.5)-(A_2+0.5)*b) *M + (A_2+0.5)*b)