consider x0, y0 being Real such that
A2: z = <*x0,y0*> and
A3: ex N being Neighbourhood of y0 st
( N c= dom (SVF1 2,(pdiff1 f,2),z) & ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF1 2,(pdiff1 f,2),z) . y) - ((SVF1 2,(pdiff1 f,2),z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) by A1, Def5;
consider N being Neighbourhood of y0 such that
A4: N c= dom (SVF1 2,(pdiff1 f,2),z) and
A5: ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF1 2,(pdiff1 f,2),z) . y) - ((SVF1 2,(pdiff1 f,2),z) . y0) = (L . (y - y0)) + (R . (y - y0)) by A3;
consider L being LINEAR, R being REST such that
A6: for y being Real st y in N holds
((SVF1 2,(pdiff1 f,2),z) . y) - ((SVF1 2,(pdiff1 f,2),z) . y0) = (L . (y - y0)) + (R . (y - y0)) by A5;
consider r being Real such that
A7: for p being Real holds L . p = r * p by FDIFF_1:def 4;
take r ; :: thesis: ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF1 2,(pdiff1 f,2),z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF1 2,(pdiff1 f,2),z) . y) - ((SVF1 2,(pdiff1 f,2),z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) )
L . 1 = r * 1 by A7
.= r ;
hence ex x0, y0 being Real st
( z = <*x0,y0*> & ex N being Neighbourhood of y0 st
( N c= dom (SVF1 2,(pdiff1 f,2),z) & ex L being LINEAR ex R being REST st
( r = L . 1 & ( for y being Real st y in N holds
((SVF1 2,(pdiff1 f,2),z) . y) - ((SVF1 2,(pdiff1 f,2),z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) ) ) ) by A2, A4, A6; :: thesis: verum