The chapters are split into sections, which, in turn, are split into subsections enumerated by two numbers: the first stands for the number of the section while the second for the number ofthe subsection itself. The same numeration is used for all kinds of statements and formulas. If we refer to statements or formulas in other chapters, we use triple numeration where the first number stands for the chapter and the other two have the same sense. The results presented in this book were discussed on the seminars at the Institute of Mathematics of Ukrainian Academy ofSciences, at the Steklov Mathematical Institute of the Academy of Sciences of the USSR, at Moscow and Tbilisi State Universities. I am deeply grateful to the heads of these seminars Professors V. K. Dzyadyk, N. P. Kor­ neichuk, S. B. Stechkin, P. L. U1yanov, and L. V. Zhizhiashvili as well as to the mem­ bers ofthese seminars that took an active part in the discussions. In TRODUCTIon It is well known for many years that every 21t -periodic summable function f(x) can be associated in a one-to-one manner with its Fourier series (1. 1) Slfl where I It = - f f(t)cosktdt 1t -It and I It - f f(t)sinktdt. 1t -It Therefore, if for approximation of a given function f(·), it is necessary to construct a sequence ofpolynomials Pn (.

1. Classes of Periodic Functions.- 1. Sets of Summable Functions. Moduli of Continuity.- 2. The Classes H?[a, b] and H?.- 3. Moduli of Continuity in the Spaces Lp. The Classes H?p.- 4. Classes of Differentiable Functions.- 5. Conjugate Functions and Their Classes.- 6. Weil-Nagy Classes.- 7. The Classes.- 8. The Classes.- 9. The Classes 35 10. Order Relation for (?, ? )-Derivatives.- 2. Integral Representations of Deviations of Linear Means Of Fourier Series.- 1. Fourier Sums.- 2. Linear Methods of Summation of Fourier Series. General Aspects.- 3. Integral Representations of ?n(f;x;?).- 4. Representations of Deviations of Fourier Sums on the Sets and.- 5. Representations of Deviations of Fourier Sums on the Sets and.- 3. Approximations by Fourier Sums in the Spaces c and l1.- 1. Simplest Extremal Problems in the Space C.- 2. Simplest Extremal Problems in the Space L1.- 3. Asymptotic Equalities for ? n(H?).- 4. Asymptotic Equalities for.- 5. Moduli of Half-Decay of Convex Functions.- 6. Asymptotic Representations for ?n(f; x) on the Sets.- 7. Asymptotic Equalities for and.- 8. Approximations of Analytic Functions by Fourier Sums in the Uniform Metric.- 9. Approximations of Entire Functions by Fourier Sums in the Uniform Metric.- 10. Asymptotic Equalities for and.- 11. Asymptotic Equalities for and.- 12. Asymptotic Equalities for and.- 13. Approximations of Analytic Functions in the Metric of the Space L.- 14. Asymptotic Equalities for and.- 15. Behavior of a Sequence of Partial Fourier Sums near Their Points of Divergence.- 4. Simultaneous Approximation of Functions and their Derivatives by Fourier Sums.- 1. Statement of the Problem and Auxiliary Facts.- 2. Asymptotic Equalities for.- 3. Asymptotic Equalities for.- 4. Corollaries of Theorems 2.1 and 3.1.- 5.Convergence Rate of the Group of Deviations.- 6. Strong Summability of Fourier Series.- 5. Convergence Rate of Fourier Series and Best Approximations in the Spaces lp.- 1. Approximations in the Space L2.- 2. Jackson Inequalities in the Space L2.- 3. Multiplicators. Marcinkiewicz Theorem. Riesz Theorem. Hardy - Littlewood Theorem.- 4. Imbedding Theorems for the Sets.- 5. Approximations of Functions from the Sets.- 6. Best Approximations of Infinitely Differentiable Functions.- 7. Jackson Inequalities in the Spaces C and Lp.- 6. Best Approximations in the Spaces C and l.- 1. Zeros of Trigonometric Polynomials.- 2. Chebyshev Theorem and de la Vallée Poussin Theorem.- 3. Polynomial of Best Approximation in the Space L.- 4. Approximation of Classes of Convolutions.- 5. Orders of Best Approximations.- 6. Exact Values of Upper Bounds of Best Approximations.- Bibliographical Notes.- References.