In the limit ρ ρ r → → 0 , Z = 1 (for finite T ); the fluid behaves like an ideal gas, a point noted several times earlier. Note additionally that the derivative D ρ ρ r Z = 3 / ( 3 − − ρ ρ r ) 2 − − 9 / ( 8 T r ) = 0 for T r = 3 ( 3 − − ρ ρ r ) 2 / 8 , and when ρ ρ r = 0 this is T r = 27 / 8 . The slope is positive or negative depending on whether T r is greater than or less than 27 / 8 , and becomes infinitely large negative as T approaches 0 .