# 5: Separation of Variables on Rectangular Domains

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In this section we shall investigate two dimensional equations defined on rectangular domains. We shall either look at finite rectangles, when we have two space variables, or at semi-infinite rectangles when one of the variables is time. We shall study all three different types of partial differental equations: parabolic, hyperbolic and elliptical.

• 5.1: Cookbook
Let me start with a recipe that describes the approach to separation of variables, as exemplified in the following sections, and in later chapters.
• 5.2: Parabolic Equation
Parabolic PDEs are used to describe a wide variety of time-dependent phenomena, including heat conduction, and particle diffusion.
• 5.3: Hyperbolic Equation
Many of the equations of mechanics are hyperbolic and the model hyperbolic equation is the wave equation.  The solutions of hyperbolic equations are "wave-like". If a disturbance is made in the initial data of a hyperbolic differential equation, then not every point of space feels the disturbance at once.
• 5.4: Laplace’s Equation
Laplace's equation are the simplest examples of elliptic partial differential equations. The solutions of Laplace's equation are the harmonic functions, which are important in many fields of science, notably the fields of electromagnetism, astronomy, and fluid dynamics, because they can be used to accurately describe the behavior of electric, gravitational, and fluid potentials. In the study of heat conduction, the Laplace equation is the steady-state heat equation.
• 5.5: More Complex Initial/Boundary Conditions
It is not always possible on separation of variables to separate initial or boundary conditions in a condition on one of the two functions. We can either map the problem into simpler ones by using superposition of boundary conditions, a way discussed below, or we can carry around additional integration constants.
• 5.6: Inhomogeneous Equations
Inhomogeneous equations can often be solved (for constant coefficient PDEs, always be solved) by finding the fundamental solution (the solution for a point source), then taking the convolution with the boundary conditions to get the solution.

Thumbnail: A visualization of a solution to the two-dimensional heat equation with temperature represented by the third dimension. Imaged used wth permission (Public Domain; Oleg Alexandrov). The heat equation is a parabolic partial differential equation that describes the distribution of heat (or variation in temperature) in a given region over time.

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