On the theory of conjugate points for parameter-invariant higher order problems in the calculus of variations

Abstract

Parameter-invariant problems of higher order in the calculus of variations have certain characteristic properties which give rise to considerable difficulties in any attempt to apply the well-known methods to develop a theory of conjugate points. In this paper a direct method is thus developedab initio which generalizes the approach of Bliss to the parameter-invariant first order problem. The Jacobi equations for the second order problem (to which the attention is initially restricted) are defined as the Euler-Lagrange equations of the accessory problem (which is non-parameter-invariant). Some properties of the Jacobi equations are investigated; in particular, so-called normal solutions of these differential equations are defined, and it is shown that all the usual existence theorems do apply to them. This fact is then used in the proof of the necessity condition of Jacobi, which is stated as usual in terms of conjugate points. While originally two equivalent definitions of the conjugate points are given in terms of solutions of the Jacobi equations these points may also be determined analytically as zeros of a certain determinant. This result is used to show that the theory is nevertheless parameter-invariant despite the’ non-parameter-invariance of the accessory problem. The final section is devoted to a description of the basic concepts of the general theory for the parameter-invariant problem of arbitrary order.