:: deftheorem defines `hpartial13| PDIFF_5:def 30 :
for f being PartFunc of (REAL 3),REAL
for D being set st f is_hpartial_differentiable`13_on D holds
for b₃ being PartFunc of (REAL 3),REAL holds
( b₃ = f `hpartial13| D iff ( dom b₃ = D & ( for u being Element of REAL 3 st u in D holds
b₃ . u = hpartdiff13 (f,u) ) ) );