Partial Differential Equations of Mathematical Physics
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If an unknown function of several variables and its partial derivatives are combined in an equation, the latter is called a partial differential equation (PDE ) . Several times above we have come across such PDEs when dealing with functions of several variables. For instance, in Sect. I.6.5.1 we derived the continuity equation for the particle density which contains its partial derivatives with respect to time t and the spatial coordinates x, y and z.