A simple mathematical model to mimic the interactions among healthy T cells, infected T cells, and free viruses is given by
dT/dt = s-rT - βTv
dT*/dt = βTv - μT*
dv/dt = kT* - cv
where state variables are the number of healthy T cells, T, the number of infected T cells, T", and the number of free viruses, v
(a) The system is nonlinear. Why?