\left( 3 \right)\mathop {\lim }\limits_{h \to 0} \frac{{\left\| {f\left( {c + hv} \right) - f\left( c \right) - hDf\left( c \right)\left( v \right)} \right\|}}{{\left| h \right| \cdot \left\| v \right\|}} = 0