The Simple model with instantaneous recycling

In the Simple model with instantaneous recycling (to be referred to as the instantaneous Simple model), we have from Eq. (8.1) 

Equation (8.6) refers to abundances in the gas and young stars. For the average abundance of a stellar population (with Z0 = 0) we have instead 

---

Some specific GCE models and related observational data 8.2 The Simple model with instantaneous recycling... 

---