Quasinormal Modes of Near-Extremal Black Holes and Black Strings in Massive Gravity Background.

Two numerical methods are used to calculate quasinormal modes~(QNMs) of near-extremal black holes/strings in the generalized spherically/cylindrically symmetric background, the Asymptotic Iteration Method~(AIM) and the Spectral Method. The numerical results confirm the accuracy of the approximate analytic formula using the P\"{o}schl-Teller potential. Our analytic formula is used to investigate the Strong Cosmic Censorship conjecture of extremal and near-extremal black holes in Ref.~\cite{Burikham:2020dfi}.