partial differential equations like the time-dependent Schr\"odinger equation $$ \begin{equation} i\hbar\frac{\partial \psi({\bf x},t)}{\partial t}= -\frac{\hbar^2}{2m}\left( \frac{\partial^2 \psi({\bf r},t)}{\partial x^2} + \frac{\partial^2 \psi({\bf r},t)}{\partial y^2}+ \frac{\partial^2 \psi({\bf r},t)}{\partial z^2}\right) + V({\bf x})\psi({\bf x},t), \tag{4} \end{equation} $$ may depend on several variables. In certain cases, like the above equation, the wave function can be factorized in functions of the separate variables, so that the Schroedinger equation can be rewritten in terms of sets of ordinary differential equations. These equations are discussed in chapter 10. Involve boundary conditions in addition to initial conditions.