theorem
  for z0 being Element of REAL 2 for N being Neighbourhood of proj(2,2).
  z0 st f is_hpartial_differentiable`12_in z0 & N c= dom SVF1(2,pdiff1(f,1),z0)
  holds for h be 0-convergent non-zero Real_Sequence,
  c be constant Real_Sequence st
rng c = {proj(2,2).z0} & rng (h+c) c= N holds h"(#)(SVF1(2,pdiff1(f,1),z0)/*(h+
c) - SVF1(2,pdiff1(f,1),z0)/*c) is convergent & hpartdiff12(f,z0) = lim (h"(#)(
  SVF1(2,pdiff1(f,1),z0)/*(h+c) - SVF1(2,pdiff1(f,1),z0)/*c))
proof
  let z0 be Element of REAL 2;
  let N be Neighbourhood of proj(2,2).z0;
  assume that
A1: f is_hpartial_differentiable`12_in z0 and
A2: N c= dom SVF1(2,pdiff1(f,1),z0);
  let h be 0-convergent non-zero Real_Sequence,
      c be constant Real_Sequence such
  that
A3: rng c = {proj(2,2).z0} & rng (h+c) c= N;
  consider x0,y0 being Element of REAL such that
A4: z0 = <*x0,y0*> by FINSEQ_2:100;
A5: pdiff1(f,1) is_partial_differentiable_in z0,2 by A1,Th10;
  then partdiff(pdiff1(f,1),z0,2) = diff(SVF1(2,pdiff1(f,1),z0),y0) by A4,
PDIFF_2:14
    .= hpartdiff12(f,z0) by A1,A4,Th6;
  hence thesis by A2,A3,A5,PDIFF_2:18;
end;
