Study of QCD thermodynamics at finite density by Taylor expansion

We discuss the phase structure and the equation of state for QCD at non-zero temperature and density. Derivatives of $\ln Z$ with respect to quark chemical potential $\mu_q$ up to fourth order are calculated for 2-flavor QCD, enabling estimates of the pressure, quark number density and associated susceptibilities as functions of $\mu_q$ via a Taylor series expansion. Also, the phase transition line for 2 and 3-flavor QCD and the critical endpoint in the $(T, \mu_q)$ plane are investigated in the low density regime.