Rankine-Hugoniot condition (source code)

= Rankine-Hugoniot condition
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Across a discontinuity $x=\sigma t$ of a <weak solution> to $u_t+f(u)_x=0$, with left and right states $u_-$ and $u_+$, conservation requires
$$
\sigma(u_+-u_-)=f(u_+)-f(u_-).
$$