`$$ \frac{dy_1}{dx} = f_1(x,y_1,y_2) \,, $$` `$$ \frac{dy_2}{dz} = f_2(x,y_1,y_2) \,, $$` subject to conditions `$y_1(x_0)=y_{1_0}$` and `$y_2(x_0)=y_{2_0}$`. This ...