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Stability criterion in 3D
\Delta t \leq \frac{1}{c}\left( \frac{1}{\Delta x^2} +
\frac{1}{\Delta y^2} + \frac{1}{\Delta z^2}\right)^{-\halfi}
For c^2=c^2(\xpoint) we must use
the worst-case value \bar c = \sqrt{\max_{\xpoint\in\Omega} c^2(\xpoint)}
and a safety factor \beta\leq 1 :
\Delta t \leq \beta \frac{1}{\bar c}
\left( \frac{1}{\Delta x^2} +
\frac{1}{\Delta y^2} + \frac{1}{\Delta z^2}\right)^{-\halfi}
