1. Bouligand, G.,Introduction à la Géométrie Infinitésimale Directe, Gauthier-Villars, Paris, France, 1932.
2. Flett, T. M.,Mathematical Analysis, McGraw-Hill, Book Company, New York, New York, 1966.
3. Flett, T. M.,On Differentiation in Normed Vector Spaces, Journal of the London Mathematical Society, Vol. 42, 1967.
4. Roetman, E. L.,Tangent Planes and Differentiation, Mathematics Magazine, Vol. 43, No. 1, 1970.
5. Hestenes, M. R.,Calculus of Variations and Optimal Control Theory, John Wiley and Sons, New York, New York, 1966.