\left( 2 \right)\mathop {\lim }\limits_{h \to 0} \frac{{\left\| {f\left( {c + hv} \right) - f\left( c \right) - Df\left( c \right)\left( {c + hv - c} \right)} \right\|}}{{\left\| {c + hv - c} \right\|}} = 0