let z be Element of REAL 2; :: thesis: for f being PartFunc of (REAL 2),REAL st f is_hpartial_differentiable`12_in z holds
hpartdiff12 f,z = partdiff (pdiff1 f,1),z,2
let f be PartFunc of (REAL 2),REAL ; :: thesis: ( f is_hpartial_differentiable`12_in z implies hpartdiff12 f,z = partdiff (pdiff1 f,1),z,2 )
assume A1: f is_hpartial_differentiable`12_in z ; :: thesis: hpartdiff12 f,z = partdiff (pdiff1 f,1),z,2
consider x0, y0 being Real such that
A2: z = <*x0,y0*> by FINSEQ_2:120;
A3: pdiff1 f,1 is_partial_differentiable_in z,2 by A1, Th10;
hpartdiff12 f,z = diff (SVF1 2,(pdiff1 f,1),z),y0 by A1, A2, Th6
.= partdiff (pdiff1 f,1),z,2 by A2, A3, PDIFF_2:14 ;
hence hpartdiff12 f,z = partdiff (pdiff1 f,1),z,2 ; :: thesis: verum