= Characteristic line for the one-dimensional wave equation
{title2=$x-ct=\mathrm{constant},\quad x+ct=\mathrm{constant}$}
= Characteristic lines for the one-dimensional wave equation
{synonym}
These <characteristic curves> are straight when <wave speed> $c$ is constant. A right-moving component is constant on $x-ct=s$, and a left-moving component on $x+ct=s$. The full <superposition> generally is not constant on either family, because the other component can vary along it. Their backwards intersections with the initial line select the endpoints used in <D'Alembert's formula>.
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