= Absorber incidence and comoving number density
{title2=$dN/dz=n_c\sigma_p c(1+z)^2/H(z)$}
For a population with <comoving number density> $n_c$ and proper interception area $\sigma_p$, the absorption incidence is the proper density multiplied by cross-section and proper light-path length $c\,dz/[H(z)(1+z)]$. The displayed relation assumes one counted absorption system per interception. Completeness, covering fraction, evolution and the distinction between observed and rest-frame wavelength separation must be stated in an inference from a spectrum.
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