  Complete spectra of the staggered Dirac operator $\Dirac$ are determined in
quenched four-dimensional $SU(2)$ gauge fields, and also in the presence of
dynamical fermions.
  Periodic as well as antiperiodic boundary conditions are used.
  An attempt is made to relate the performance of multigrid (MG) and conjugate
gradient (CG) algorithms for propagators with the distribution of the
eigenvalues of~$\Dirac$.
  The convergence of the CG algorithm is determined only by the condition
number~$\kappa$ and by the lattice size.
  Since~$\kappa$'s do not vary significantly when quarks become dynamic,
  CG convergence in unquenched fields can be predicted from quenched
simulations.
  On the other hand, MG convergence is not affected by~$\kappa$ but depends on
the spectrum in a more subtle way.
