As we will see shortly, there are differential equations for describing how this function changes in space and time. Such a differential equation is not an ordinary differential equation. It can’t be, because it doesn’t depend on just one independent variable. In fact, it depends on four of them: x, y, z, and t. It’s a new kind of beast—a partial differential equation, so called because each of its independent variables plays its own “part” in causing change to occur. Partial differential equations are much richer than ordinary differential equations. They describe continuous systems moving
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