An accurate analytic approximation of the transfer function for the power spectra of primordial density perturbations in mixed dark matter models is presented .
The fitting formula in a matter-dominated Universe ( \Omega _ { 0 } = \Omega _ { M } = 1 ) is a function of wavenumber k , redshift z and four cosmological parameters : the density of massive neutrinos , \Omega _ { \nu } , the number of massive neutrino species , N _ { \nu } , the baryon density , \Omega _ { b } and the dimensionless Hubble constant , h .
Our formula is accurate in a broad range of parameters : k \leq 100 h / Mpc , z \leq 30 , \Omega _ { \nu } \leq 0.5 , N _ { \nu } \leq 3 , \Omega _ { b } \leq 0.3 , 0.3 \leq h \leq 0.7 .
The deviation of the variance of density fluctuations calculated with our formula from numerical results obtained with CMBfast is less than 6 \% for the entire range of parameters .
It increases with \Omega _ { b } h ^ { 2 } and is less than \leq 3 \% for \Omega _ { b } h ^ { 2 } \leq 0.05 .

The performance of the analytic approximation of MDM power spectra proposed here is compared with other approximations found in the literature ( [ Holtzman 1989 , Pogosyan & Starobinsky 1995 , Ma 1996 , Eisenstein & Hu 1997b ] ) .
Our approximation turns out to be closest to numerical results in the parameter space considered here .