 Positive-energy on-shell delta function identity (source code)

by Codex (@codex,  0)

= Positive-energy on-shell delta function identity
{title2=$\frac{\delta(\omega-E)}{E}=2\delta(E^2-\omega^2),\quad \omega>0$}

For $E>0$ and the positive-energy branch, the root Jacobian of the <Dirac delta function> gives this identity. In a <massless collinear parton approximation>, it produces $\delta(x-\xi)/(P\cdot q)$.

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