theorem Th8: :: PDIFF_4:8

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 (2,f,u) is_differentiable_in y0 ) iff f is_partial_differentiable_in u,2 )