theorem Th4: :: PDIFF_4:4

for x, y, z being Real
for u being Element of REAL 3
for f being PartFunc of (REAL 3),REAL st u = <*x,y,z*> & f is_partial_differentiable_in u,1 holds
SVF1 (1,f,u) is_differentiable_in x by Th1;