The pseudospectrum and transient of Kaluza-Klein black holes in Einstein-Gauss-Bonnet gravity

The spectrum and dynamical instability, as well as the transient effect of the tensor perturbation for the so-called Maeda-Dadhich black hole, a type of Kaluza-Klein black hole, in Einstein-Gauss-Bonnet gravity have been investigated in framework of pseudospectrum. We cast the problem of solving quasinormal modes (QNMs) in AdS-like spacetime as the linear evolution problem of the non-normal operator in null slicing by using ingoing Eddington-Finkelstein coordinates. In terms of spectrum instability, based on the generalised eigenvalue problem, the QNM spectrum and $\epsilon$-pseudospectrum has been studied, while the open structure of $\epsilon$-pseudospectrum caused by the non-normality of operator indicates the spectrum instability. In terms of dynamical instability, we introduce the concept of the distance to dynamical instability, which plays a crucial role in bridging the spectrum instability and the dynamical instability. We calculate such distance, named the complex stability radius, as parameters vary. Finally, we show the behaviour of the energy norm of the evolution operator, which can be roughly reflected by the three kinds of abscissas in context of pseudospectrum, and find the transient growth of the energy norm of the evolution operator.