let x0, y0, r be Real; :: thesis: for z being Element of REAL 2
for f being PartFunc of REAL 2, REAL st z = <*x0,y0*> & f is_partial_differentiable`2_in z holds
( r = diff (SVF2 f,z),y0 iff ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) )
let z be Element of REAL 2; :: thesis: for f being PartFunc of REAL 2, REAL st z = <*x0,y0*> & f is_partial_differentiable`2_in z holds
( r = diff (SVF2 f,z),y0 iff ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) )
let f be PartFunc of REAL 2, REAL ; :: thesis: ( z = <*x0,y0*> & f is_partial_differentiable`2_in z implies ( r = diff (SVF2 f,z),y0 iff ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) ) )
set F1 = SVF2 f,z;
assume that
A1: z = <*x0,y0*> and
A2: f is_partial_differentiable`2_in z ; :: thesis: ( r = diff (SVF2 f,z),y0 iff ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) )

hereby :: thesis: ( ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) implies r = diff (SVF2 f,z),y0 )

assume AB: r = diff (SVF2 f,z),y0 ; :: thesis: ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) )
AA: f is_partial_differentiable_in z,2 by A2, Lem4;
SVF2 f,z is_differentiable_in y0 by A1, AA, Lem2;
then consider N being Neighbourhood of y0 such that
A4: ( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) by AB, FDIFF_1:def 6;
thus ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) by A1, A4; :: thesis: verum

end;

given x1, y1 being Real such that C1: ( z = <*x1,y1*> & ex N being Neighbourhood of y1 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y1) = (L . (y - y1)) + (R . (y - y1)) ) ) ) ) ; :: thesis: r = diff (SVF2 f,z),y0
D1: y1 = y0 by C1, A1, FINSEQ_1:98;
f is_partial_differentiable_in z,2 by A2, Lem4;
then SVF2 f,z is_differentiable_in y0 by A1, Lem2;
hence r = diff (SVF2 f,z),y0 by D1, C1, FDIFF_1:def 6; :: thesis: verum