Stability implies robust convergence of a class of diagonalization-based iterative algorithms

Solving wave equations in a time-parallel manner is challenging, and the algorithm based on the block $\alpha$-circulant preconditioning technique has shown its advantage in many existing studies (where $\alpha\in(0, 1)$ is a free parameter). Considerable efforts have been devoted to exploring the spectral radius of the preconditioned matrix and this leads to many case-by-case studies depending on the used time-integrator. In this paper, we propose a unified way to analyze the convergence via directly studying the error of the algorithm and using the stability of the time integrator. Our analysis works for all one-step time-integrators and two exemplary classes of two-step time-integrators: the parameterized Numerov methods and the parameterized two-stage hybrid methods. The main conclusion is that the global error satisfies $\|{\textbf{err}}^{k+1}\|_{{\mathbf W}, 2}\leq \frac{\alpha}{1-\alpha}\|{\mathbf{err}}^{k}\|_{{\mathbf W}, 2}$ provided that the time-integrator is stable, where $k$ is the iteration index and for any vector $v$ the norm $\|v\|_{{\mathbf W}, 2}$ is defined by $\|v\|_{{\mathbf W}, 2}=\|{\mathbf W}v\|_2$ with ${\mathbf W}$ being a matrix depending on the space discretization matrix only. Even though we focus on wave equations in this paper, the proposed argument is directly applicable to other evolution problems.