Group velocity from a period-dependent phase velocity
= Group velocity from a period-dependent phase velocity
{title2=$u=c/[1+(P/c)\,dc/dP]$}
Since $k=\omega/c(\omega)$, <differentiation> gives $u=(dk/d\omega)^{-1}=c/[1-(\omega/c)\,dc/d\omega]$. Using the <period> $P=2\pi/\omega$ gives the displayed formula. A positive <period> slope makes the <group velocity> smaller than the <phase velocity> on this branch. A slope read from a graph produces an approximate local result, not a globally fitted <dispersion> law.