Some variational principles for problems in hydrodynamic and hydromagnetic stability

Briefly, the boundary value problem L(u) = 0 on xi < x < x2, where L is a differential operator, is said to be self-adjoint if for any two functions u and v satisfying the specified boundary conditions at xi and x2 the integral from Xi to xt of vL(u) -uL(v) vanishes. See 8, p. 59 or [9, for a more detailed discussion of self-adjoint boundary value problems.