1. J.J. Stoker, DifferentialGeometryWiley-Interscience, New York (1969).
2. I.M. Gelfand and S.V. FominCalculus of VariationsPrentice-Hall, Englewood Cliffs, NJ (1963).
3. S.B. Alexander, I.D. Berg and R.L. BishopThe Riemannian obstacle problemIllinois Journal of Mathematics, 31 (1987), 167–184.
4. F. Albrecht and I.D. BergGeodesics in euclidean spacewith analyticobstacleProceedings of the American Mathematical Society, 113 (1991), 201–207.
5. M.P. de Carmo, RiemannianGeometryBirkhauser, Boston (1992).