CONTENTS
1 TRIGONOMETRIC FOURIER SERIES Page 1. 1:
Periodic Functions, 1. 2: Harmonies, 3. 3: Trigonometric
Polynomials and Series, 6. 4: A More Precise Terminology.
Integrability. Series of Functions, 8. 5: The Basic Trigo-
nometric System. The Orthogonality of Sines and Cosines,
10. 6: Fourier Series for Functions of Period 21:, 12. 7:
Fourier Series for Functions Defined on an Interval of
Length 21:, 15. 8: Right-hand and Left-hand Limits. Jump
Discontinuities, 17. 9: Smooth and Piecewise Smooth
Functions, 18. 10: A Criterion for the Convergence of
Fourier Series, 19. 11: Even and Odd Functions, 21. 12:
Cosine and Sine Series, 22. 13: Examples of Expansions in
Fourier Series, 24. 14: The Complex Form of a Fourier
Series, 32. 15: Functions of Period 21, 35. Problems, 38.
2 ORTHOGONAL SYSTEMS Page 41. 1: Definitions, 41.
2: Fourier Series with Respect to an Orthogonal System, 42.
3: Some Simple Orthogonal Systems, 44. 4: Square Inte-
grable Functions. The Schwarz Inequality, 50. 5: The
Mean Square Error and its Minimum, 51. 6: Bessel’s
Inequality, 53. 7: Complete Systems. Convergence in the
Mean, 54. 8: Important Properties of Complete Systems, 57.
9: A Criterion for the Completeness of a System, 58. ”'10:
The Vector Analogy, 60. Problems, 63.
3 CONVERGENCE OF TRIGONOMETRIC FOURIER
SERIES Page 66. l : A Consequence of Bessel’s Inequality,
66. 2: The Limit as n—->oo of the Trigonometric Integrals
L”f(x) cos nx dx and 1370:) sin nx dx, 67. 3: Formula for
the Sum of Cosines. Auxiliary Integrals, 71. 4: The
Integral Formula for the Partial Sum of a Fourier Series, 72.
5: Right-Hand and Left-Hand Derivatives, 73. 6: A Sufficient
Condition for Convergence of a Fourier Series at a Con-
tinuity Point, 75. 7: A Sufficient Condition for Convergence
of a Fourier Series at a Point of Discontinuity, 77. 8:
Generalization of the Sufficient Conditions Proved in Secs.
6 and 7, 78. 9: Convergence of the Fourier Series of a
Piecewise Smooth Function (Continuous or Discontinuous),
79. 10: Absolute and Uniform Convergence of the Fourier
Series of a Continuous, Piecewise Smooth Function of Period
21:, 80. 11: Uniform Convergence of the Fourier Series of a
Continuous Function of Period 21: with an Absolutely
Integrable Derivative, 82. 12: Generalization of the Results
of Sec. 11, 85. 13: The Localization Principle, 90. 14:
Examples of Fourier Series Expansions of Unbounded Func-
tions, 91. 15: A Remark Concerning Functions of Period
21, 94. Problems, 94.
4 TRIGONOMETRIC SERIES WITH DECREASING
COEFFICIENTS Page 97. l: Abel’s Lemma, 97. 2:
Formula for the Sum of Sines. Auxiliary Inequalities, 98.
3: Convergence of Trigonometric Series with Monotonically
Decreasing Coefficients, 100. *4: Some Consequences of the
Theorems of Sec. 3, 103. 5: Applications of Functions of a
Complex Variable to the Evaluation of Certain Trigono-
metric Series, 105. 6: A Stronger Form of the Results of
Sec. 5, 108. Problems, 112.
5 OPERATIONS ON FOURIER SERIES Page 115. 1:
Approximation of Functions by Trigonometric Polynomials,
115. 2: Completeness of the Trigonometric System, 117. 3:
Parseval’s Theorem. The Most Important Consequences of
the Completeness of the Trigonometric System, 119. *4:
Approximation of Functions by Polynomials, 120. 5:
Addition and Subtraction of Fourier Series, Multiplication
of a Fourier Series by a Number, 122. ”'6: Products of
Fourier Series, 123. 7: Integration of Fourier Series, 125.
8: Differentiation of Fourier Series. The Case of a Con-
tinuous Function of Period 21:, 129. I"9: Difi‘erentiation of
Fourier Series. The Case of a Function Defined on the
Interval [— 1r, 1:], 132. ‘10: Difi‘erentiation of Fourier Series.
The Case of a Function Defined on the Interval [0, 1:], 137.
11: Improving the Convergence of Fourier Series, 144. 12:
A List of Trigonometric Expansions, 147. 13: Approximate
Calculation of FOurier Coefiicients, 150. Problem, 152.
6 SUMMATION OF TRIGONOMETRIC FOURIER
SERIES Page 155. 1: Statement of the Problem, 155. 2:
The Method of Arithmetic Means, 156. 3: The Integral
commrs ix
Formula for the Arithmetic Mean of the Partial Sums of a
Fourier Series, 157. 4: Summation of Fourier Series by the
Method of Arithmetic Means, 158. 5: Abel’s Method of
Summation, 162. 6: Poisson’s Kernel, 163. 7: Application
of Abel’s Method to the Summation of Fourier Series, 164.
Problems, 170.
7 DOUBLE FOURIER SERIES. THE FOURIER
INTEGRAL Page 173. 1: Orthogonal Systems in Two
Variables, 173. 2: The Basic Trigonometric System in Two
Variables. Double Trigonometric Fourier Series, 175. 3:
The Integral Formula for the Partial Sums of a Double
Trigonometric Fourier Series. A Convergence Criterion,
178. 4: Double Fourier Series for a Function with Different
Periods in x and y, 180. 5: The Fourier Integral as
a Limiting Case of the Fourier Series, 180. 6: Improper
Integrals Depending on a Parameter, 182. 7: Two Lemmas,
185. 8: Proof of the Fourier Integral Theorem, 188. 9:
Different Forms of the Fourier Integral Theorem, 189. *10:
The Fourier Transform, 190. ’11: The Spectral Function,
193. Problems, 195.
8 BESSEL FUNCTIONS AND FOURIER-BESSEL SERIES
Page 197. l: Bessel’s Equation, 197. 2: Bessel Functions of
The First Kind of Nonnegative Order, 198. 3: The Gamma
Function, 20]. 4: Bessel Functions of the First Kind of
Negative Order, 202. 5: The General Solution of Bessel’s
Equation, 203. 6: Bessel Functions of the Second Kind, 204.
7: Relations between Bessel Functions of Different Orders,
205. 8: Bessel Functions of the First Kind of Half-Integral
Order, 207. 9: Asymptotic Formulas for the Bessel Functions
208. 10: Zeros of the Bessel Functions and Related Func-
tions, 213. 11: Parametric Form of Bessel’s Equation, 215.
12: Orthogonality of the Functions Jpotx), 216. 13: Evalua-
tion of the Integral £x1p2(Xx)dx, 218. I'14: Bounds for the
Integral L’ xJPZQx) dx, 219. 15: Definition of Fourier-Bessel
Series, 220. 16: Criteria for the Convergence of Fourier-
Bessel Series, 221. ‘17: Bessel’s Inequality and its Conse-
quences, 223. ‘18: The Order of Magnitude of the Coeffi-
cients which Guarantees Uniform Convergence of a Fourier-
Bessel Series, 225. *19: The Order of Magnitude of the
Fourier-Bessel Coefficients of a Twice Differentiable Function,
228. ‘20: The Order of Magnitude of the Fourier-Bessel
Coefficients of a Function Which ix com
Times, 231. ‘21: Term by Term Difi’erentiation of Fourier-
Bessel Series, 234. 22: Fourier-Bessel Series of the Second
Type, 237. I"23: Extension of the Results of Secs. 17—21 to
Fourier-Bessel Series of the Second Type, 239. 24: Fourier-
Bessel Expansions of Functions Defined on the Interval
[0, I], 241. Problems, 243.
9 THE EIGENFUNCTION METHOD AND ITS APPLICA-
TIONS T0 MATHEMATICAL PHYSICS Page 245.
Part I: THEORY. 1: The Gist of the Method, 245. 2: The
Usual Statement of the Boundary Value Problem, 250. 3:
The Existence of Eigenvalues, 250. 4: Eigenfunctions and
Their Orthogonality, 251. 5: Sign of the Eigenvalues, 254.
6: Fourier Series with Respect to the Eigenfunctions, 255.
7: Does the Eigenfunction Method Always Lead to a Solution
of the Problem ?, 258. 8: The Generalized Solution, 261. 9:
The Inhomogeneous Problem, 264. 10: Supplementary
Remarks, 266. Part II: APPLICATIONS, 268. 11: Equation of
a Vibrating String, 268. 12: Free Vibrations of a String,
269. 13: Forced Vibrations of a String, 273. 14: Equation
of the Longitudinal Vibrations of a Rod, 275. 15: Free
Vibrations of a Rod, 277. 16: Forced Vibrations of a Rod,
280. 17: Vibrations of a Rectangular Membrane, 282. 18:
Radial Vibrations of a Circular Membrane, 288. 19:
Vibrations of a Circular Membrane (General Case), 291. 20:
Equation of Heat Flow in a Rod, 296. 21: Heat Flow in a
Rod with Ends Held at Zero Temperature, 297. 22: Heat
Flow in a Rod with Ends Held at Constant Temperature,
299. 23: Heat Flow in a Rod Whose Ends are at Specified
Variable Temperatures, 301. 24. Heat Flow in a Rod Whose
Ends Exchange Heat Freely with the Surrounding Medium,
301. 25: Heat Flow in an Infinite Rod, 306. 26: Heat Flow
in a Circular Cylinder Whose Surface is Insulated, 310. 27:
Heat Flow in a Circular Cylinder Whose Surface Exchanges
Heat with the Surrounding Medium, 312. 28: Steady-State
Heat Flow in a Circular Cylinder, 313. Problems, 316.
ANSWERS T0 PROBLEMS Page 319.
BIBLIOGRAPHY Page 331.
INDEX Page 333.
