theorem Th9: :: PDIFF_4:9

for f being PartFunc of (REAL 3),REAL
for u being Element of REAL 3 holds
( ex x0, y0, z0 being Real st
( u = <*x0,y0,z0*> & SVF1 (3,f,u) is_differentiable_in z0 ) iff f is_partial_differentiable_in u,3 )