The eigenvalues of linear, regular, two point boundary value problems
depend continuously on the problem. In the important self-adjoint case
studied by NAIMARK and WEIDMANN this dependence is differentiable and
the derivatives of the eigenvalues with respect to a given parameter:
an endpoint, a boundary condition, a coefficient, or the weight funct
ion, are found. Monotone properties of the eigenvalues with respect to
the coefficients and the weight function are established without usin
g the variational (min-max) characterization.