df(x,y)(h) = ^(x,y)hl + J^ dif(x,y)(h) = d(df(xly)(h)) =
(x, y )h2
+
— (x, y Ih, + — (x, y )h3
ć?2f , a
-T ^1
a*2 1
d2f
dXd/
hih2 +
■ hi +
a2f
ayax
T