def aic(k,lnL):
    """
    Akaike Information Criterion -- the lowest value is preferred.

    INPUT:
        k - number of parameters in the model
        lnL - log likelihood

    NOTES:
        The relative likelihood is definied as exp(Delta AIC/2) Grunblatt et al. 2015

    https://en.wikipedia.org/wiki/Akaike_information_criterion
    """
    return 2.*(k-lnL)