# 12.1 The Dirichlet conditions: Chapter 12 Fourier series Advantages: (1)describes functions that are not everywhere continuous and/or differentiable. (2)represent.

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12.1 The Dirichlet conditions: Chapter 12 Fourier series Advantages: (1)describes functions that are not everywhere continuous and/or differentiable. (2)represent the response of a system to a period input and depend on the frequency of the input (3)using in string vibration, light scattering, input signal transmission in electronic circuit (1)The function must be periodic. (2)It must be single-valued and continuous, except possibly at a finite number of finite discontinuities. (3)It must have only a finite number of maxima and minima within one period. (4)The integral over one period of a function must converge.

all functions may be written as the sum of an odd and an even part Chapter 12 Fourier series chosen as the sum of a cosine series chosen as the sum of a sine series orthogonal properties: the length of a period is L:

Fourier series expansion of the function f(x) is Chapter 12 Fourier series

Ex: Express the square-wave function as a Fourier series Chapter 12 Fourier series f(t) is an odd function, so only the sine term survives

(1) At a point of finite discontinuity,, the Fourier series converges to (2) At a discontinuity, the Fourier series representation of the function will overshoot its value. It never disappears even in the limit of an infinite number of terms. This behavior is known as Gibb’s phenomenon. Chapter 12 Fourier series 1 term 2 terms 3 terms 20 terms overshooting 12.4 Discontinuous functions

Chapter 12 Fourier series 12.5 Non-periodic functions: period=L, no particular symmetry period=2L, antisymmetry; odd fun period=2L, symmetry; even fun

Ex. :Find the Fourier series of Chapter 12 Fourier series (1) make the function periodic and symmetric

(2) make the function periodic and antisymmetric Chapter 12 Fourier series

Integration and differentiation (1) The Fourier series of f(x) is integrated term by term then the resulting Fourier series converges to the integral of f(x). (2) f(x) is a continuous function of x and is periodic then the Fourier series that results from differentiating term by term converges to f(x). Chapter 12 Fourier series Ex: Find the Fourier series of Sol:from the previous example integrate (1) term by term put (3) into (2)

12.7 Complex Fourier series Complex Fourier series expansion is: Chapter 12 Fourier series

Ex: Find a complex Fourier series for in the range

general proof: Chapter 12 Fourier series 12.8 Parseval’s theorem:

Ex: Using Parseval’s theorem and the Fourier series for evaluate the sum Chapter 12 Fourier series

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