def frohner_cor(sig1,sig2,n1,n2):
    """
    Takes cross-sections [barns] and atom densities [atoms/barn] for 
    two thicknesses of the same sample, and returns extrapolated cross
    section according to Frohner.

    Parameters
    ----------
    sig1 : array_like
        Cross section of the thinner of the two samples.
    sig2 : array_like
        Cross section of the thicker of the two samples.
    n1 : float
        Atom density of the thinner sample
    n2 : float 
        Atom density of the thicker sample

    Returns
    -------
    sig0 : array_like
        The extrapolated cross section from sig1 and sig2
    """

    return (n2*sig1-n1*sig2)/(n2-n1)