On the zero-stability of multistep methods on smooth nonuniform grids
For numerical analysts and practitioners using multistep methods with step size control, this provides theoretical justification for zero stability under smooth grid variations, addressing a long-standing gap in the literature.
The paper proves that linear multistep methods are zero stable on smooth nonuniform grids where step sizes change smoothly, with adjacent step size ratios of 1 + O(N^{-1}), provided the deformation map is C^2 and N is sufficiently large. This extends classical constant step size theory to practical nonuniform grids.
In order to be convergent, linear multistep methods must be zero stable. While constant step size theory was established in the 1950's, zero stability on nonuniform grids is less well understood. Here we investigate zero stability on compact intervals and smooth nonuniform grids. In practical computations, step size control can be implemented using smooth (small) step size changes. The resulting grid $\{t_n\}_{n=0}^N$ can be modeled as the image of an equidistant grid under a smooth deformation map, i.e., $t_n = Φ(τ_n)$, where $τ_n = n/N$ and the map $Φ$ is monotonically increasing with $Φ(0)=0$ and $Φ(1)=1$. The model is justified for any fixed order method operating in its asymptotic regime when applied to smooth problems, since the step size is then determined by the (smooth) principal error function which determines $Φ$, and a tolerance requirement which determines $N$. Given any strongly stable multistep method, there is an $N^*$ such that the method is zero stable for $N>N^*$, provided that $Φ\in C^2[0,1]$. Thus zero stability holds on all nonuniform grids such that adjacent step sizes satisfy $h_n/h_{n-1} = 1 + \mathrm O(N^{-1})$ as $N\rightarrow\infty$. The results are exemplified for BDF-type methods.