  The fluctuation exchange (FLEX) approximation is applied to study the
Holstein-Hubbard model. Due to the retarded nature of the phonon-mediated
electron-electron interaction, neither fast Fourier transform (FFT) nor
previously developed NRG methods for Hubbard-type purely electronic models are
applicable, while brute force solutions are limited by the demands on
computational time and storage which increase rapidly at low temperature $T$.
Here,we describe a new numerical renormalization group (NRG) technique to solve
the FLEX equations efficiently. Several orders of magnitude of CPU time and
storage can be saved at low $T$ ($\sim 80K$). To test our approach, we compare
our NRG results to brute force calculations on small lattices at elevated
temperatures. Both s-wave and d-wave superconducting phase diagrams are then
obtained by applying the NRG approach at low $T$. The isotope effect for s-wave
pairing is BCS-like in a realistic phonon frequency range, but vanishes at
unphysically large phonon frequency ($\sim $ band width). For d-wave pairing,
the isotope exponent is negative and small compared to the typical observed
values in non-optimally doped cuprates.
