Some convergence and stability results for rational approximations of holomorphic semigroups are presented. The approximations are related to time discrete schemes with nonuniform stepsize to a linear evolution equation of parabolic type in a Banach space. Our theorems cover not only the backward Euler but also the Crank-Nicolson and forward Euler schemes.Let -A be the infinitesimal generator of a holomorphic semigroup {e -to }t>o in a Banach space X, and let r(z) be a rational function of order p > 1, that is,As is well-known, if r(z) is strongly A(0)-acceptable for some 0 E (0, ir/2), for example r(z) = (1 + z) -1 , we havewhere II -II denotes the operator norm in X; r a positive constant; n a positive integer; and C a positive constant which is independent of A, n and T. For more detail and other results, we refer to [7], [8] and [15]. The main purpose of the present paper is to extend (1) to the case where the stepsize T is not uniform. Specifically, taking positive constants Tl , T2,. . . ,^ , we introduce a general partition t, = Ti + T2 • + T^ of the time variable. Then we shall estimate
Il r(TnA)r(Tn-iA) ... r (T1A) -e -t " A IIand derive convergence rate in terms of p, {Tj }^ 1 and tn . As will discuss below, in doing so, we meet a new issue. Namely, we can not avoid the ratio T/r0 , where T = max Tj, To = min Tj.1 0)
under various assumptions on A and r(z).Our problem is closely related to error estimate for a time discretization to the evolution equationwhere u = u(t) E D(A) and a E X denote the unknown function and the initial value, respectively. The approximation un E D(A) of u(t) at the time level t = to is given by
un = r(TA)r(Tr-1 A) ... r(Tl A)a. (4)Especially, when r(z) = (1 + z) -1 , (4) implies the backward Euler scheme with the nonuniform stepsize;un -un-1 + Au n = 0 (n>1), uo = a, Tom, and, by virtue of (2), we obtain the error estimatewhere II • 11 means the norm in X. Furthermore, when r(z) = (1 -z/2)(1 + z/2) -1 and r(z) = 1 -z, the scheme (4) corresponds to the Crank-Nicolson and forward Euler schemes with the nonuniform stepsize, respectively:un -un _1 1 + 2 (Au n + Au-i) = 0 (n > 1), uo = a; T n un -un-i + Aun-1 = 0 (n > 1), uo = a,
TnRational Approximation of Semigroups 325 a...