  The main aim of this study is the comparison of gravitational waveforms
obtained from numerical simulations which employ different numerical evolution
approaches and different wave-extraction techniques. For this purpose, we
evolve an oscillating, non-rotating polytropic neutron-star model with two
different approaches: a full nonlinear relativistic simulation (in three
dimensions) and a linear simulation based on perturbation theory. The
extraction of the gravitational-wave signal is performed with three methods:
The gauge-invariant curvature-perturbation theory based on the Newman-Penrose
scalar $\psi_4$; The gauge-invariant Regge-Wheeler-Zerilli-Moncrief
metric-perturbation theory of a Schwarzschild space-time; Some generalization
of the quadrupole emission formula.
