  In underdoped cuprates, T_c is thought to be determined by Cooper pair phase
fluctuations because of the small superfluid density n_s. Experimentally, T_c
is found to scale with n_s. The fluctuation-exchange approximation (FLEX) in
its standard form fails to predict this behavior of T_c since it does not
include phase fluctuations. We therefore extend the FLEX to include them
selfconsistently. We present results for T_c[n_s,x], where x is the doping.
