Acknowledgments

We thank NIH/National Center for Research Resources (P41RR13622), NSF and the Spanish Government (BIO99-0651-CO2-01) for support, and C.-K. Peng, A.L. Goldberger and Y. Ashkenazy for helpful discussions. When concluding our work, we became aware of an independent study by J.W. Kantelhardt et. al[66], where similar issues are discussed. We thank J.W. Kantelhardt and A. Bunde for sending us their preprint before publication.