Open Access. The Published version of this work has been reproduced here in accordance with the Publisher’s self archiving and copyright policies.
This article may be downloaded for personal use only. Any other use requires prior permission of Cambridge University Press. This work was published in Acta Numerica, 11, 2002 p. 341-434 published by Cambridge University Press and can be found at Cambridge Journals online http://dx.doi.org/10.1017/S0962492902000053