0.1$ [@Fodor:2004nz]-[@Rischke:2003mt].

The rapid rise of the entropy was predicted long before QCD by Hagedorn due to the observed exponential rise of the number of hadron resonances [@Karsch:2003zq]. However, the saturation of the number of degrees of freedom near $n_{QCD}$ is a unique feature of QCD. Even though the entropy density approaches the ideal, weakly interacting plasma limit, lattice calculations of correlators show that the QGP is far from ideal below $3T_c$. The nonideal nature of this strongly coupled QGP is also seen from the deviation of the pressure, $P(T)$, and energy density $\epsilon(T)$ from the Stefan Boltzmann limit as shown in Fig.(\[qgpfig1\]) from [@Fodor:2004nz].

The equation of state of the QGP, $P_{QCD}(T)$, is the bulk thermodynamic property that can be investigated experimentally via “barometric” observables. A measure of its stiffness is given by the speed of sound squared, $c_s^2=dP/d\epsilon=d\log T/d\log\sigma=(3+d\log n/d\log T)^{-1}$ shown in Fig.(\[qgpfig2\]). Note that $c_s^2$ drops rapidly below $1/3$ as the effective number of degrees of freedom drops when $T$ approaches $T_c$. This softening of the QGP equation of state near $T_c$ is a key feature can be looked for in the collective hydrodynamic flow patterns produced when the plasma expands.

![A recent Lattice QCD calculation 