English: Linear operators are compatible with addition, i.e. the sum ${\displaystyle v_{3}=v_{1}+v_{2}}$ is maintained by the linear operator ${\displaystyle f}$ via ${\displaystyle f(v_{3})=f(v_{1})+f(v_{2})}$. This property can be condensed in ${\displaystyle f(v_{1}+v_{2})=f(v_{1})+f(v_{2})}$.