The gravitational waves emitted just after the binary black hole collision can be well approximated by the quasinormal mode(QNM). In general relativity(GR), QNM of a black hole is completely determined by the mass and the angular momentum due to the uniqueness theorem. Thus, we can test GR by checking whether QNM frequencies are close to the GR prediction or not in future observation. To do this, we should know QNM frequencies for non-GR gravity in advance. In this talk, we discuss a systematic way to compute QNM frequencies if the basic equation for gravitational waves is close that for GR. For simplicity, we focus on the Schwarzschild case with some correction terms. We also show that our method can reproduce some known results.
references: arXiv:1901.01265, arXiv:1906.05155, arXiv:2001.09613