To find a unique solution for Partial Differential Equations (PDEs) like the diffusion or wave equation, both boundary conditions (BCs), defined on the spatial surface $S$ for all time, and initial conditions (ICs), defined across the volume $V$ at $t=t_0$, must be specified. Common BCs include Dirichlet (fixing the function value $u$ ), Neumann (fixing the normal derivative $n \cdot \nabla u$ or flux), and the generalized Robin condition. The number of ICs needed is determined by the highest-order time derivative. The uniqueness of these solutions is often established through energy methods. Solutions that do not depend on time are called stationary states, which satisfy Poisson's equation ( $\nabla^2 u=-\rho$ ); these require only BCs, with Laplace's equation ( $\nabla^2 u=0$ ) being a special case. Importantly, while solutions to linear problems are generally unique, Poisson's equation with only Neumann conditions results in a solution unique only up to an arbitrary constant, requiring a consistency condition for existence.

Key takeaways

1. Necessity for Unique Solutions
   • Conditions are essential: To find a unique solution to a PDE (like the diffusion or wave equation), a sufficient number of boundary conditions (BCs) and initial conditions (ICs) must be specified.
   • Parameter space: The conditions are required on the entire boundary of the relevant parameter space, which includes the spatial boundary (surface $S$ ) for $t>t_0$ and the entire volume ( $V$ ) at the initial time $t=t_0$.
2. Types of Conditions
   • Initial Condition (IC): Specifies the state of the system at the initial time $t_0$ across the entire volume $V$.
     • The number of ICs needed is equal to the order of the highest time derivative (e.g., Diffusion is first-order, needs one IC; Wave is second-order, needs two ICs).
   • Boundary Conditions (BCs): Specify the system's behavior on the spatial surface $S$ for $t>t_0$.
     • Dirichlet BC: Specifies the value of the function $u$ on the boundary ( $u=f$ ). Physically, this means keeping the quantity fixed.
     • Neumann BC: Specifies the value of the normal derivative $n \cdot \nabla u$ on the boundary. Physically, this often relates to a known net current or flux out of the volume.
     • Robin BC: A general linear combination of Dirichlet and Neumann conditions ( $\alpha u+ \beta n \cdot \nabla u=k)$.
3. Uniqueness and Energy Methods
   • Uniqueness Proof: The uniqueness of solutions for time-dependent PDEs (like the wave equation) with given BCs and ICs is often proven using energy methods.
   • Energy Functional: By defining a positive-definite energy functional $E[w]$ for the difference $w$ between two hypothetical solutions, it can be shown that $E[w]$ must be zero for all time, proving $w=0$.
4. Stationary States and Related Equations
   • Stationary States: Solutions that do not depend on time ( $\partial_t u=0$ ). They require only BCs, no ICs.
     • Stationary solutions to the diffusion and wave equations satisfy Poisson's equation ( $\nabla^2 u=-\rho$ ).
   • Laplace's Equation: A special case of Poisson's equation with a zero source term ( $\nabla^2 u=$ 0 ), describing stationary states with no sources.
   • Uniqueness Exception (Poisson's Equation): When all BCs for Poisson's equation are Neumann conditions, the solution is not unique (it can be shifted by an arbitrary constant $C$). A consistency condition between the source $\rho$ and the boundary flux $g$ must be met for any solution to exist.
5. Linearization
   • Linear Differential Equation: An equation of the form $\hat{L} u=f$, where $\hat{L}$ is a linear differential operator and $f$ does not depend on $u$.
   • Linearization: Any non-linear PDE can be linearized by studying small deviations ( $u= u_0+\varepsilon v$ ) around a known solution $u_0$. The resulting PDE for the deviation $v$ is linear.

🫧Cue Column

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1. From Extensive Properties to the Continuity Equation
2. Derivation of the Diffusion and Heat Equations from the Continuity Principle
3. The Wave Equation-Derivation and Physical Applications and Wave Speed Determination
4. Boundary and Initial Conditions for Partial Differential Equations-Types and Uniqueness and Stationary States
5. Fluid Momentum and the Continuity Equation-Derivation of the Cauchy and Navier-Stokes Equations
6. The Wave Equation-Derivation and Physical Applications and Wave Speed Determination
7. Boundary and Initial Conditions for Partial Differential Equations-Types and Uniqueness and Stationary States </aside>

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