A solution of Problem 19 was carried through for general second order nonlinear elliptic equations in 2 dimensions by S. Bernstein beginning in 1904 (see Section 7). His methods gave rise to essential techniques of estab-lishing a priori estimates for solutions and their derivatives, in particular, using the linearization of nonlinear equations in a neighborhood of a solution. (For a detailed discussion of developments arising from Hilbert problems 19 and 20, see the articles by J. Serrin and G. Stampacchia in the volume"Mathematical developments arising from Hilbert problems'' published by the AMS in 1976.)