$\renewcommand{\Re}{\operatorname{Re}}$ $\renewcommand{\Im}{\operatorname{Im}}$ $\newcommand{\erf}{\operatorname{erf}}$ $\newcommand{\dag}{\dagger}$ $\newcommand{\const}{\mathrm{const}}$ ##1.2. Initial and Boundary Value Problems --- > 1. [Problems for PDEs](#sect-1.2.1) > 2. [Notion of 'well-posedness'](#sect-1.2.2) ###Problems for PDEs We know that solutions of ODEs typically depend on one or several constants. For PDEs situation is more complicated. Consider simplest equations \begin{gather} u\_x =0, \label{eq-1.2.1}\\\\ v\_{xy}=0 \label{eq-1.2.2} \end{gather} with $u=u(x,y)$ and $v=v(x,y)$. Equation (\ref{eq-1.2.1}) could be treaded as an ODE with respect to $x$ and its solution is a constant but this is not a genuine constant as *it is constant only with respect to $x$ and can depend on other variables*; so $u(x,y)=\phi(y)$. Then for solution of (\ref{eq-1.2.2}) we have $v_y= \phi(y)$ where $\phi$ is an arbitrary function of one variable and it could be considered as ODE with respect to $y$; then $(v-g(y))_y=0$ where $g(y)=\int \phi(y)\,dy$, and therefore $v-g(y)=f(x)\implies v(x,y)=f(x)+g(y)$ where $f,g$ are arbitrary functions of one variable. Considering these equations again but assuming that $u=u(x,y,z)$, $v=v(x,y,z)$ we arrive to $u=\phi(y,z)$ and $v=f(x,z)+g(y,z)$ where $f,g$ are arbitrary functions of two variables. Solutions to PDEs typically depend not on several arbitrary constants but on one or several arbitrary functions of $n-1$ variables. For more complicated equations this dependance could be much more complicated and implicit. To select a right solutions we need to use some extra conditions. The sets of such conditions are called *Problems*. Typical problems are - IVP (*Initial Value Problem*): one of variables is interpreted as *time* $t$ and conditions are imposed at some moment; f.e. $u|_{t=t_0}=u_0$; - BVP (*Boundary Value Problem*) conditions are imposed on the boundary of the spatial domain $\Omega$: f.e. $u|_{\partial\Omega}=\phi$ where $\partial\Omega$ is a boundary of $Omega$; - IVBP (*Initial-Boundary Value Problem* aka *mixed problem*): one of variables is interpreted as *time* $t$ and some conditions are imposed at some moment but other conditions are imposed on the boundary of the spatial domain. **Remark 1.** In the course of ODEs students usually consider IVP only. F.e. for the second-order equation like \begin{equation\*} u\_{xx}+ a_1 u\_{x}+a\_2 u=f(x) \end{equation\*} such problem is $u|\_{x=x\_0}=u\_0$, $u\_x|\_{x=x_0}=u\_1$. However one could consider BVPs like \begin{gather\*} (\alpha\_1 u_x+\beta\_1 u)|\_{x=x\_1}=\phi\_1,\\\\ (\alpha\_2 u\_x+\beta\_2 u)|\_{x=x\_2}=\phi\_2, \end{gather\*} where solutions are sought on the interval $[x\_1,x\_2]$. Such are covered in advanced chapters of some of ODE textbooks (but not covered by a typical ODE class). We will need to cover such problems later in this Textbook. ###Notion of "well-posedness" We want that our PDE (or the system of PDEs) together with all these conditions satisfied the following requirements: - Solutions must exist for all right-hand expressions (in equations and conditions); - Solution must be unique; - Solution must depend on this right-hand expressions continuously. Such problems are called *well-posed*. PDEs are usually studied together with the problems which are well-posed for these PDEs. Different types of PDEs "admit" different problems. Sometimes however one needs to consider *ill-posed* problems. ________ [$\Leftarrow$](./S1.1.html)  [$\Uparrow$](../contents.html)  [$\Rightarrow$](./S1.3.html)