THE FRACTAL STRUCTURE OF THE UNIVERSE Paul H. COLEMAN~ and Luciano PIETRONEROC aSterrewacht Leiden, University of Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands bKapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands cDipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro, 00185 Rome, Italy NORTH-HOLLAND