= Finite propagation in porous-medium diffusion
In a <porous medium equation> with exponent $m>1$, the effective <diffusion coefficient> vanishes at zero density. A nonnegative compactly supported source profile can therefore retain a finite front for every finite positive time. For example, the <radial cubic-diffusion source profile> has support radius proportional to $t^{1/6}$. This notion concerns a moving diffusion front; it does not require the constant characteristic cone used for a <wave equation>.
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