theorem Th6: :: PDIFF_2:6

for f being PartFunc of (REAL 2),REAL
for z being Element of REAL 2 holds
( ex x0, y0 being Real st
( z = <*x0,y0*> & SVF1 (2,f,z) is_differentiable_in y0 ) iff f is_partial_differentiable_in z,2 )