3. Positivity and boundedness of solutions
In this section, we first state positivity and boundedness of solutions
of system (2) without proof as it is obvious. These are very important
so far as the validity of the model is concerned.
Lemma 1 . All solutions\((x_{1}\left(t\right),x_{2}\left(t\right),y\left(t\right))\)of system (2) with initial values\((x_{10},x_{20}\), \(y_{0})\ \epsilon\mathbb{\text{\ R}}_{+}^{3},\)