English: Nonlinear operators might not be compatible with addition, i.e. we find arguments ${\displaystyle v_{3}=v_{1}+v_{2}}$ with ${\displaystyle f(v_{3})\neq f(v_{1})+f(v_{2})}$. This property can be condensed in ${\displaystyle f(v_{1}+v_{2})\neq f(v_{1})+f(v_{2})}$ for some vectors ${\displaystyle v_{1},v_{2}}$.