Solana hard disk equation of state

The Solana equation of state for hard disks (2-dimensional hard spheres) is given by Eq. 26 of

$Z={\frac {1+\eta ^{2}/8-\eta ^{4}/10}{(1-\eta )^{2}}}$

where the packing fraction is given by $\eta =\pi \rho \sigma ^{2}/4$ where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma} is the diameter of the disks.