= Velocity-space diffusion
{title2=$\partial_tf=\partial_v(D(v)\partial_vf)$}
Random changes of particle <velocity> described by a <diffusion equation> in <velocity space>. For constant $D$, $\langle(\Delta v)^2\rangle=2D\Delta t$. With zero boundary flux it conserves number. For a number-normalized one-dimensional distribution, its <kinetic energy> production is $d\mathcal K/dt=-m\int vD(v)f'(v)\,dv$, which reduces to $mnD$ for constant $D$. Variable $D$ must remain inside the derivative.
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