# 4: Fourier series and PDEs

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• 4.10: Dirichlet problem in the circle and the Poisson kernel
• 4.1: Boundary value problems
• 4.2: The Trigonometric Series
• 4.3: More on the Fourier Series
• 4.4: Sine and cosine series
You may have noticed by now that an odd function has no cosine terms in the Fourier series and an even function has no sine terms in the Fourier series. This observation is not a coincidence. Let us look at even and odd periodic function in more detail.
• 4.5: Applications of Fourier series
• 4.6: PDEs, separation of variables, and the heat equation
Let us recall that a partial differential equation or PDE is an equation containing the partial derivatives with respect to several independent variables. Solving PDEs will be our main application of Fourier series. A PDE is said to be linear if the dependent variable and its derivatives appear at most to the first power and in no functions. We will only talk about linear PDEs. Together with a PDE, we usually have specified some boundary conditions.
• 4.7: One dimensional wave equation
• 4.8: D’Alembert solution of the wave equation
We have solved the wave equation by using Fourier series. But it is often more convenient to use the so-called d’Alembert solution to the wave equation3. While this solution can be derived using Fourier series as well, it is really an awkward use of those concepts. It is easier and more instructive to derive this solution by making a correct change of variables to get an equation that can be solved by simple integration.
• 4.9: Steady state temperature and the Laplacian
Suppose we have an insulated wire, a plate, or a 3-dimensional object. We apply certain fixed temperatures on the ends of the wire, the edges of the plate, or on all sides of the 3-dimensional object. We wish to find out what is the steady state temperature distribution. That is, we wish to know what will be the temperature after long enough period of time.
• 4.E: Fourier series and PDEs (Exercises)
These are homework exercises to accompany Libl's "Differential Equations for Engineering" Textmap. This is a textbook targeted for a one semester first course on differential equations, aimed at engineering students. Prerequisite for the course is the basic calculus sequence.

## Contributors

This page titled 4: Fourier series and PDEs is shared under a not declared license and was authored, remixed, and/or curated by Jiří Lebl.