2 “is \ | ' NORTH-HOLLAND a4 DISCRETE MATHEMATICS Editor-in-Chief Peter L.H ammer, Piscataway (NJ) Advisory Editors C. Berge, Paris V.L. Klee, Seattle (WA) J.H. van Lint, Eindhoven A.J. Hoffman, R.C. Mullin, Waterloo Yorktown Heights (NY) V.T. Sos, Budapest Board of Editors M.S. Aigner, Berlin D. Foata, Strasbourg I. Rival, Ottawa B. Alspach, Burnaby A.S. Fraenkel, Rehovot A. Rosa, Hamilton G.E. Andrews, Univ. Park (PA) P. Frankl, Tokyo 1.G. Rosenberg, Montreal A. Barlotti, Firenze A.M. Frieze, Pittsburgh (PA) S. Rudeanu, Bucharest C. Benzaken, Grenoble I.M. Gessel, Waltham (MA) H. Sachs, IlImenau J.-C. Bermond, R.L. Graham, La Jolla (CA) J. Schonheim, Tel-Aviv Sophia-Antipolis A. Hajnal, Budapest N.J.A. Sloane, N.L. Biggs, London F. Harary, Las Cruces (NM) Florham Park (NJ) 8. Bollobas, Memphis (TN) D.M. Jackson, Waterloo C. Thomassen, Lyngby R.A. Brualdi, Madison (WI) J. Kahn, Piscataway (NJ) W.T. Trotter, Tempe (AZ) T.H. Brylawski, G.O.H. Katona, Budapest W.T. Tutte, Newmarket Chapel Hill (NC) D.J. Kleitman, D.J.A. Welsh, Oxford P.J. Cameron, London Cambridge (MA) D.B. West, Urbana (IL) P. Camion, Le Chesnay A.V. Kostochka, Urbana (IL) R. Wille, Darmstadt G. Chartrand, Kalamazoo (MI) P. Leroux, Montreal D.R. Woodall, Nottingham V. Chvatal, Piscataway (NJ) L. Lovasz, Redmond (WA) H.P. Yap, Singapore Editorial Manager Carolyn Sterner Issue Manager Mick van Gijlswijk Publication Information: Discrete Mathematics (ISSN 0012-365X). For 2000 volumes 210-225 are scheduled for publication. A combined subscription to Discrete Mathematics and Discrete Applied Mathematics (Vols. 98-106) at reduced rate is available. Subscription prices are available upon request from the Publisher or from the Regional Sales Office nearest you or from this journal’s website (http://www.elsevier.nl/locate/disc). Further information is available on this journal and other Elsevier Science products through Elsevier’s website (http://www.elsevier.nl). 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Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. Abstracted/Indexed in: ACM Computing Reviews, Anbar Electronic Intelligence, Cambridge Scientific Abstracts, Current Contents: Physical, Chemical & Earth Sciences, International Abstracts in Operations Research, Mathematical Reviews, PASCAL, Science Citation Index, Zentralblatt fiir Mathematik. © The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper) Editor-in-Chief Peter L. Hammer, RUTCOR, Rutgers, the State University of New Jersey, 640 Bartholomew Road, Piscataway, NJ 08854-8003, USA Advisory Editors C. Berge, E.R. Combinatoire, Centre de Mathématique Sociale, 54 Boulevard Raspail, 75270 Paris Cedex 06, France A.J. Hoffman, Mathematical Sciences Department, IBM Thomas Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA V.L. Klee, Department of Mathematics, University of Washington, Seattle, WA 98195, USA R.C. Mullin, Department of Combinatorics & Optimization, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 V.T. Sos, Mathematical Institute, Elke TTK Analisis 1, Muzeum Krt. 6—8, H-Budapest 8, Hungary J.H. van Lint, Technische Universiteit, Insulindelaan 2, 5612 AZ Eindhoven, Netherlands Board of Editors M.S. Aigner, FB Mathematik, WE2, Freie Universitat Berlin, Arnimallee 3, 14195 Berlin 33, Germany B. Alspach, Department of Mathematics & Statistics, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6 G.E. Andrews, Department of Mathematics & Statistics, Pennsylvania State University, University Park, PA 16802, USA A. Barlotti, Istituto Matematico “Ulisse Dini”, Viale Morgagni 67/A, I-50134 Firenze, Italy C. Benzaken, /nstitute of Advanced Mathematics, Scientific and Medical, University of Grenoble, BP 53X, 38041 Grenoble Cedex, France J.-C. Bermond, Project SLOOP, I3S-INRIA, 2004 Route des Lucioles, BP 93, F-06902 Sophia-Antipolis, France N.L. Biggs, Department of Mathematics, London School of Economics, Houghton Street, London WC2A 2AE, UK B. Bollobas, Department of Mathematical Sciences, University of Memphis, Campus Box 526429, Memphis, TN 38152-6429, USA R.A. Brualdi, Department of Mathematics, University of Wisconsin-Madison, 480 Lincoln Drive, Madison, WI 53706, USA T.H. Brylawski, Department of Mathematics, University of North Carolina, Chapel Hill, NC 27514, USA P.J. Cameron, School of Mathematical Sciences, Queen Mary College, University of London, Mile End Road, London El 4NS, UK P. Camion, INRIA, Domaine de Volucean-Rocquencourt, BP 105, Le Chesnay Cedex 78153, France G. Chartrand, Department of Mathematics, Western Michigan University, Kalamazoo, MI 49008, USA V. Chvatal, Department of Computer Science, Rutgers, the State University of New Jersey, Hill Center, Piscataway, NJ 08855, USA D. Foata, Département Mathématique, Universite Louis Pasteur, 7 rue René Descartes, F-67084 Strasbourg, France A.S. Fraenkel, Department of Applied Mathematics, Weizmann Institute of Science, IL-76100 Rehovot, Israel P. Frankl, Shibuya-Ku, Higashi 1-10-30301, Tokyo 150, Japan A.M. Frieze, Department of Mathematics, Carnegie Mellon University, Pittsburgh, PA 15213, USA I.M. Gessel, Department of Mathematics, Brandeis University, P.O. Box 9110, Waltham, MA 02254-9110, USA R.L. Graham, University of California-San Diego, CSE APM, La Jolla, CA 92093-0114, USA A. Hajnal, Mathematical Institute, Hungarian Academy of Science, Realtanoda u. 13—15, H-1053 Budapest, Hungary F. Harary, Department of Computer Science, New Mexico State University, Las Cruces, NM 88003, USA D.M. Jackson, Combinatorics & Optimization, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 J. Kahn, Department of Mathematics, Rutgers, the State University of New Jersey, Hill Center, Piscataway, NJ 08855, USA G.O.H. Katona, Matematik Kutato Intez, Magyar Tudomanyos Akad, Realtanoda u. 13-15, H-1053 Budapest, Hungary D.J. Kleitman, Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA A.V. Kostochka, Department of Mathematics, University of Illinois at Urbana- Champaign, 1409 West Green Street, Urbana, IL 61801-2975, USA P. Leroux, Département de Mathématiques et Informatiques, Universite du Quebec a Montréal, CP 888 Succ. A., Montreal, Quebec, Canada H3C 3P8 L. Lovasz, Microsoft Research, One Microsoft Way, Redmond, WA 98052, USA I. Rival, Department of Computer Science, University of Ottawa, Ottawa, Ont., Canada KIN 6N5 A. Rosa, Department of Mathematics, McMaster University, Hamilton, Ont., Canada L8S 4K1 I.G. Rosenberg, Mathématiques et Statistiques, Université de Montréal, B.P. 6218, Succ. A., Montreal, Quebec, Canada H3C 3J7 S. Rudeanu, /nstitutei de Mathematica, University of Bucharest, Str. Academiei 14, 70109 Bucaresti, Romania H. Sachs, TH/Sekt. Mathematik, Rechentechnik und Kybernetik, Postfach 327, 98693 Ilmenau, Germany J. Schonheim, Department of Mathematics, Tel Aviv University, Ramat Aviv, IL-Tel Aviv, Israel N.J.A. Sloane, AT&T Research Labs., Room C233, P.O. Box 971, 180 Park Ave, Florham Park, NJ 07932-0971, USA C. Thomassen, Mathematical Institute, Technical University of Denmark, Building 303, DK-2800 Lyngby, Denmark W.T. Trotter, Department of Mathematics, Arizona State University, Tempe, AZ 85287, USA W.T. Tutte, 1/5] Manderston Road, Newmarket, Suffolk CB8 ONS, UK D.J.A. Welsh, Mathematical Institute, University of Oxford, 24—29 St. Giles, Oxford OX] 3LB, UK D.B. West, Department of Mathematics, University of Illinois at Urbana-Champaign, 273 Altgeld Hall, 1409 West Green Street, Urbana, IL 61801-2975, USA R. Wille, Fachbereich Mathematik, Technische Hochschule Darmstadt, Schlossgarten- strasse 7, 64289 Darmstadt, Germany D.R. Woodall, Department of Mathematics, University of Nottingham, University Park, Nottingham NG7 2RD, UK H.P. Yap, Department of Mathematics, National University of Singapore, Singapore 0511, Singapore