  We investigate the third-order density perturbation and the one-loop
correction to the linear power spectrum in the dark-energy cosmological model.
Our main interest is to understand the dark-energy effect on baryon acoustic
oscillations in a quasi-nonlinear regime ($k \approx 0.1h$/Mpc). Analytical
solutions and simple fitting formulae are presented for the dark-energy model
with the general time-varying equation of state $w(a)$. It turns out that the
power spectrum coincides with the approximate result based on the EdS (Einstein
de-Sitter) model within 1% for $k<0.4h/$Mpc at $z=0$ in the WMAP (Wilkinson
Microwave Anisotropy Probe) 5yr best-fitting cosmological model, which suggests
that the cosmological dependence is very small.
