Numerical stability in the
presence of variable coefficients
Ernst Hairer and Arieh Iserles
Abstract. The main concern of this paper is
with the stable discretisation of linear partial differential
equations of evolution with time-varying coefficients.
We commence by demonstrating that an approximation of the
first derivative by a skew-symmetric matrix is fundamental
in ensuring stability for many differential equations of
evolution. This motivates our detailed study of skew-symmetric
differentiation matrices for univariate finite-difference
methods.
We prove that, in order to sustain a skew-symmetric
differentiation matrix of order p>=2, a grid must satisfy 2p-3
polynomial conditions. Moreover, once it satisfies these
conditions, it supports a banded skew-symmetric differentiation
matrix of this order and of the bandwidth $2p-1$ which can be
derived in a constructive manner.
Some applications require not just skew-symmetry but also
that the growth in the elements of the differentiation matrix
is at most linear in the number of unknowns. This is always
true for our tridiagonal matrices of order 2 but need not be
true otherwise, a subject which we explore further.
Another subject which we examine is the existence and practical
construction of grids that support skew-symmetric differentiation
matrices of a given order. We resolve this issue completely for
order-two methods.
We conclude the paper with a list of open problems and their
discussion.
Key Words. Stable discretisation, linear PDEs,
skew-symmetric differentiation matrices, order conditions,
banded differentiation matrices.