n-Widths in Approximation Theory

Introduction . . . . . . . . Basic Properties of n-Widths . Properties of d * * * * * * * * * * 9 n 15 2. Existence of Optimal Subspaces for d * n n 17 3. Properties of d * * * * * * 20 4. Properties of b * * * * * * n 5. Duality Between d and d * * 27 n 7. Some Relationships Between dn(T), dn(T) and bn(T) . 32 37 Notes and References . . . . .