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A separable partial differential equation is one that can be broken into a set of separate equations of lower dimensionality by a method of separation of variables. This generally relies upon the problem having some special form or symmetry. In this way, the partial differential equation can be solved by solving a set of simpler PDEs, or even ordinary differential equations if the problem can be broken down into one-dimensional equations.
The most common form of separation of variables is simple separation of variables in which a solution is obtained by assuming a solution of the form given by a product of functions of each individual coordinate. There is a special form of separation of variables called R {\displaystyle R} -separation of variables which is accomplished by writing the solution as a particular fixed function of the coordinates multiplied by a product of functions of each individual coordinate. Laplace's equation on R n {\displaystyle {\mathbb {R} }^{n}} is an example of a partial differential equation which admits solutions through R {\displaystyle R} -separation of variables; in the three-dimensional case this uses 6-sphere coordinates.