Liu hard disk equation of state

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

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

Failed to parse (syntax error): {\displaystyle Z = \frac{1 + \eta^2/8 + \eta^4/18- \4 \eta^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.