© 1988 by Institute of Mathematics and its Applications
Systems with Time Delay in the Calculus of Variations: A Variational Approach
Centro de Investigación en Matemáticas, A. C. Apartado Postal 402, Guanajuato, Gto., 36000, Mexico
In this paper, we deal with the fixed-endpoint problem in the calculus of variations involving a delay in the phase coordinates. Necessary conditions in the form of a maximum principle are well known and, hence, conditions equivalent to those of Euler, Legendre, and Weierstrass. However, no results seem to exist for sufficiency, or for a corresponding Jacobi condition. We derive necessary and sufficient conditions in terms of the first and second variations, extending the clssical results for the delay-free case. This is obtained directly, that is, without referring to concepts such as conjugate points, fields of extremals, Riccati equations, or the Hamilton-Jacobi partial differential equation. The first-order condtion is then characterized in terms of the Euler equation, together with smoothness properties of solutions. Several examples illustrate the usefulness of the conditions obtained.