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Discretization of derivatives at the boundary (2)
\frac{u_{-1}^n - u_1^n}{2\Delta x} = 0
- Problem: u_{-1}^n is outside the mesh (fictitious value)
- Remedy: use the stencil at the boundary to eliminate u_{-1}^n ;
just replace u_{-1}^n by u_{1}^n
\begin{equation}
u^{n+1}_i = -u^{n-1}_i + 2u^n_i + 2C^2
\left(u^{n}_{i+1}-u^{n}_{i}\right),\quad i=0 \end{equation}
