2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/179736We consider the variation of the surface spanned by closed strings in a spacetime manifold. Using the Nambu-Goto string action, we induce the geodesic surface equation, the geodesic surface deviation equation which yields a Jacobi field, and we define the index form of a geodesic surface as in the case of point particles to discuss conjugate strings on the geodesic surface.4 pages, more references are addedMathematical PhysicsHigh Energy Physics - Theorytext