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_partial_differentiable`2_in z holds
ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
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
ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
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 ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) )
assume that
A1: z = <*x0,y0*> and
A2: f is_partial_differentiable`2_in z ; :: thesis: ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) )
consider x1, y1 being Real such that
A3: ( z = <*x1,y1*> & SVF2 f,z is_differentiable_in y1 ) by A2, Def7;
SVF2 f,z is_differentiable_in y0 by A1, A3, FINSEQ_1:98;
hence ex N being Neighbourhood of y0 st
( N c= dom (SVF2 f,z) & ex L being LINEAR ex R being REST st
for y being Real st y in N holds
((SVF2 f,z) . y) - ((SVF2 f,z) . y0) = (L . (y - y0)) + (R . (y - y0)) ) by FDIFF_1:def 5; :: thesis: verum