def R(a0,a1,a2,a3,a4,T):
    """
    Troscompt et al (2009) coefficients using Faure et al (2004) equation:
    log10(R) = sum(a_n T^{-n/6})
    where n=0..4, R is presumably cm^3 s^-1
    """
    return a0 + a1*T**(-1./6.) + a2*T**(-2./6.) + a3*T**(-3./6.) + a4*T**(-4./6.)