, ,

.

, Df = 0, f . . . . n = 1 Df ³0 , . . . . . . , , , , z = f (x, y), f (x, ) - . . , z = F (x, ), F (x, ) - ( "", "").

, f . , . . .
( . super - ) - , Df £ 0. f - , f . ., . . . : n =2

n = 3

( r - ). G DV = - 2pr DU = - 4pr , , r ³ 0 . . r < 0.
. . , , ( , ), .
.: . ., , .-., 1937.

: