In 1912, S. Bernstein, in the
first part of his memoir [2] devoted to the boundary value problems arising in
calculus of variations, established sufficient conditions for the unique solvability of
the Dirichlet problem for the equation y′′= f(t,y,y′). Our aim is to present a result
which extends the scope of the Bernstein theorem and to show that the
generalization obtained can be carried over (with only minor adjustments in the
proof) to the case of all important boundary value problems which arise in
applications.