let x0, y0 be Real; :: thesis: for z being Element of REAL 2
for f being PartFunc of REAL 2, REAL st z = <*x0,y0*> & f is_hpartial_differentiable`22_in z holds
SVF2 (pdiff2 f,z),z is_differentiable_in y0
let z be Element of REAL 2; :: thesis: for f being PartFunc of REAL 2, REAL st z = <*x0,y0*> & f is_hpartial_differentiable`22_in z holds
SVF2 (pdiff2 f,z),z is_differentiable_in y0
let f be PartFunc of REAL 2, REAL ; :: thesis: ( z = <*x0,y0*> & f is_hpartial_differentiable`22_in z implies SVF2 (pdiff2 f,z),z is_differentiable_in y0 )
assume that
A1: z = <*x0,y0*> and
A2: f is_hpartial_differentiable`22_in z ; :: thesis: SVF2 (pdiff2 f,z),z is_differentiable_in y0
consider x1, y1 being Real such that
A4: ( z = <*x1,y1*> & ex N being Neighbourhood of y1 st
( N c= dom (SVF2 (pdiff2 f,z),z) & ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF2 (pdiff2 f,z),z) . y) - ((SVF2 (pdiff2 f,z),z) . y1) = (L . (y - y1)) + (R . (y - y1)) ) ) by A2, Def6;
y0 = y1 by A1, A4, FINSEQ_1:98;
hence SVF2 (pdiff2 f,z),z is_differentiable_in y0 by A4, FDIFF_1:def 5; :: thesis: verum