Acknowledgement

Ian H. Sloan School of Mathematics University of New South Wales Sydney, NSW 2052 Australia I . SloanQunsw. ed u . au h ttp ://w w w .m ath s.u n sw .ed u .a u /~ s lo a n /

Robert S. Womersley School of Mathematics University of New South Wales Sydney, NSW 2052 Australia R .WomersleyQunsw. ed u . au h ttp ://w w w .m ath s .u n sw .ed u .au /~ rsw /