# 3: Trigonometric Fourier Series

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“Ordinary language is totally unsuited for expressing what physics really asserts, since the words of everyday life are not sufficiently abstract. Only mathematics and mathematical logic can say as little as the physicist means to say.” Bertrand Russell (1872-1970)

• 3.1: Introduction to Fourier Series
From the study of the heat equation and wave equation, we have found that there are infinite series expansions over other functions, such as sine functions. We now turn to such expansions and in the next chapter we will find out that expansions over special sets of functions are not uncommon in physics. But, first we turn to Fourier trigonometric series.
• 3.2: Fourier Trigonometric Series
Our goal is to find the Fourier series representation given f(x) . Having found the Fourier series representation, we will be interested in determining when the Fourier series converges and to what function it converges.
• 3.3: Fourier Series Over Other Intervals
In many applications we are interested in determining Fourier series representations of functions defined on intervals other than [0,2π] . In this section we will determine the form of the series expansion and the Fourier coefficients in these cases.
• 3.4: Sine and Cosine Series
In the last two examples (f(x)=|x| and f(x)=x on [−π,π] ) we have seen Fourier series representations that contain only sine or cosine terms. As we know, the sine functions are odd functions and thus sum to odd functions. Similarly, cosine functions sum to even functions. Such occurrences happen often in practice. Fourier representations involving just sines are called sine series and those involving just cosines (and the constant term) are called cosine series.
• 3.5: Solution of the Heat Equation
• 3.6: Finite Length Strings
We now return to the physical example of wave propagation in a string.
• 3.7: The Gibbs Phenomenon
We have seen from the Gibbs Phenomenon when there is a jump discontinuity in the periodic extension of a function, whether the function originally had a discontinuity or developed one due to a mismatch in the values of the endpoints. The Fourier series has a difficult time converging at the point of discontinuity and these graphs of the Fourier series show a distinct overshoot which does not go away. This is called the Gibbs phenomenon.
• 3.8: Problems

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