Liu hard disk equation of state

The Liu equation of state for hard disks (2-dimensional hard spheres) is given by Eq. 1 of

Failed to parse (lexing error): Z = \frac{1 + \eta^2/8 + \eta^4/18- \4eta^4/21}{(1-\eta)^2}

where the packing fraction is given by $\eta = \pi \rho \sigma^2 /4$ where $\sigma$ is the diameter of the disks.