def Planck_frequency(nu, T):
return <NUM_LIT:2>*hPlanck*nu**<NUM_LIT:3>/c_light**<NUM_LIT:2>/(exp(hPlanck*nu/kBoltzmann/T)-<NUM_LIT:1>)<EOL>
The Planck function B(nu,T):
    the flux density for blackbody radiation in frequency space
    nu is frequency in 1/s
    T is temperature in Kelvin

    Formula (3.1) from Raymond Pierrehumbert, "Principles of Planetary Climate"