:: deftheorem Def6 defines `| NDIFF_3:def 6 :
for F being RealNormSpace
for f being PartFunc of REAL, the carrier of F
for X being set st f is_differentiable_on X holds
for b<sub>4</sub> being PartFunc of REAL, the carrier of F holds
( b<sub>4</sub> = f `| X iff ( dom b₄ = X & ( for x being Real st x in X holds
b₄ . x = diff (f,x) ) ) );