Gravitational-Wave Extraction from Neutron Star Oscillations: comparing   linear and nonlinear techniques
  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. 
Luca Baiotti, Sebastiano Bernuzzi, Giovanni Corvino, Roberto De   Pietri, Alessandro Nagar