UDF for DO model
 

I use the DO model of Fluent. The DO model considers the radiative transfer equation :





dI(r,s)/ds + (a + σs)I(r,s) = an2σT4/π+(σs/4π)I(r,s) Φ(s.s) dΩ (13.3-1)



Is there an UDF which calculate the first part of this equation dI(r,s)/ds ?
Thank you very much