def Prefactor(flux,index,emin,emax,escale):
    """Compute the prefactor at the energy escale knowing
    the flux and index between emin and emax"""

    Denomin = pow(emax,-abs(index)+1.) -pow(emin,-abs(index)+1.)
    return flux*(-abs(index)+1)*pow(escale,-abs(index)) / Denomin