We distinguish also between linear and non-linear differential equation where for example $$ \begin{equation} \frac{dy}{dt}=g^3(t)y(t), \tag{5} \end{equation} $$ is an example of a linear equation, while $$ \begin{equation} \frac{dy}{dt}=g^3(t)y(t)-g(t)y^2(t), \tag{6} \end{equation} $$ is a non-linear ODE.