Zegel    Mathematical Institute Newsletter
September 2002
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The Mathematical Institute newsletter is maintained by Jan-Hendrik Evertse. The newsletter will be issued the first week of each month, except for July and August. The electronic version of the newsletter can be found on
(go to the homepage of the Mathematical Institute and click on "Newsletter").

Copy for the newsletter of October, such as announcements, personal notes, visits to conferences, guests, lectures of general interest, should be sent by e-mail to evertse@math.leidenuniv.nl before September 27. To avoid any inconvenient translating or typing work, copy should be submitted in English plaintext.

Press release Universiteit Leiden, August 24, 2002.

  • On August 24, 2002, the Universiteit Leiden has issued a press release, concerning a project supervised by Hendrik Lenstra and Bart de Smit, which is related to Maurits Escher's famous lithograph 'Print Gallery.'
    This lithograph shows a boy in a print gallery looking at a picture showing the gallery itself. In the center of the lithograph, Escher had left open a white hole which has intrigued many people. As it turned out, there is a hidden mathematical structure behind Escher's lithograph, which Hendrik Lenstra managed to unravel. This structure, which is based on some transformations from complex analysis, was intuitively discovered by Escher, who had no more than a secondary school knowledge of mathematics. The mathematical structure indicates how the hole left open by Escher should look like, but apparently Escher himself did not know how to handle the hole.
    After two years of drawing and programming work, which was performed in addition to the usual teaching and research tasks, the group of Lenstra and de Smit managed to make a computer animation of Escher's lithograph with the filled hole. There were important contributions by the Ph.D. students Joost Batenburg and Richard Groenewegen, the graphic artists Jacqueline Hofstra and Hans Richter, and the systems management of our institute (Martijn Feleus, Lennert Buytenhek, Willem Jan Palenstijn).
    For more information on this project, see
    http://www.math.leidenuniv.nl/~desmit/escherinfo/ (with some high resolution pictures).
    Further, there is an illustrated article in the New York Times of July 30, 2002: http://www.nytimes.com/2002/07/30/science/30ESCH.html (for subscribers only).
    The full Dutch text of the press release is given below.


    Leidse wiskundigen vullen gat in Escher's 'prentententoonstelling'

    De Leidse getaltheoreticus en Spinozaprijs-winnaar Hendrik Lenstra heeft de wiskundige structuur achter Eschers bekende litho 'Prentententoonstelling' ontrafeld. Op deze ingenieuze prent staat een jongeman in een galerij naar een prent te kijken waarop die galerij zelf afgebeeld staat. Het midden van de prent heeft Escher open gelaten. Lenstra heeft laten zien dat Escher, die geen hogere wiskundige opleiding had dan de middelbare school, in deze prent op een unieke wiskundige structuur gestuit is, die de weg wijst naar de opvulling van het midden. In de zomer van 2002 slaagden Leidse mathematici er na twee jaar teken- en programmeerwerk in Eschers prent te completeren. De opgevulde prent, en vele variaties en animaties zijn te zien op de website van het project 'Escher en het droste-effect':


    Het gat in de prent uit 1956 heeft veel mensen geïntrigeerd, onder wie Douglas Hofstadter in zijn boek Gödel, Escher, Bach. Maar het werd tot dusver niet op een overtuigende manier ingevuld. In 2000 werd wiskundige Lenstra door dit mysterieuze gat gegrepen. Hij wist het wiskundige probleem in een paar dagen op te lossen. Hiermee lag de weg open om de prent daadwerkelijk op te vullen. Tussen colleges en lopend onderzoek door is met programmeerwerk en bij-dragen van grafici Eschers litho uitgebreid en zijn computeranimaties gemaakt. Bovendien heeft het nieuwe wiskundige begrip ook vele nieuwe varianten van Eschers prent opgeleverd.

    De wiskundige structuur achter Eschers prent blijkt die van een 'elliptische kromme' te zijn. Deze krommen spelen een essentiële rol bij het vinden van priemfactoren van grote getallen, cryptografie en het bewijs van de Laatste Stelling van Fermat. Deze onderwerpen behoren tot het onderzoeksgebied van Spinozaprijs-winnaar Hendrik Lenstra. Hij is hoogleraar wiskunde aan de Universiteit Leiden en de University of California in Berkeley (USA). Het project is in Leiden uitgevoerd door getaltheoreticus Bart de Smit en wiskunde- en informaticastudent Joost Batenburg.

Announcement for all computer users of the Mathematical Institute

  • In order to help organize the work of our helpdesk and system administration we have a new system to keep track of questions and problems about the use of the computers. The system helps to keep a history of solved problems, to detect what kind of problems are the most time consuming and to allow the already overloaded systems group to work with fewer interruptions.

    The main point is: ALL requests should be made by email to


    for general problems and to


    for account creation and deletion.
    Please include a meaningful subject line: not "question" but "I cannot print".
    You will be answered also by email.

    If you want, you can view the entire to-do list that is being worked on at
    http://support    (username: math, password: helpme)
    Further information about the system, and why we need it, can be found at http://help/rt.html
    Documentation about our computer system is at http://help/

    Thanks for your cooperation.

    Bart de Smit, Computer committee MI
    Martijn Feleus, Systems Administration

Agenda and announcements

  • Per September 1, our building Niels Bohrweg 1 is open on Mondays-Fridays from 7:30 am-8:00 pm.

  • Dr. H. Wisbrun (former Mathematics didactician at ICLON) has taken a new position as advisor of higher education at ICLON. His position as Mathematics didactician is taken over by P. Kop.

  • Prof.dr. D.L. Donoho (Stanford, USA) is the Kloosterman Professor 2002. He will give a special lecture on Monday September 9.
    Title: Data! Data! Data! Challenges and Opportunities from the coming "Data Deluge"
    Time: Monday September 9, 3:30-4:30 pm
    Place: Lorentz Center

  • Workshop on high-dimensional data, p >> n in mathematical statistics and bio-medical applications.
    Lorentz Center, Leiden; September 9-20, 2002.
    Organizers: D. L. Donoho (Stanford, USA), J. C. van Houwelingen (LUMC, Leiden), S. A. van de Geer (Leiden).

  • Instructional conference Explicit Algebraic Number Theory (Stieltjesweek).
    Lorentz Center, Leiden; September 23-26, 2002.
    Organizers: H.W. Lenstra, P. Stevenhagen (Leiden).

  • NWO-OTKA workshop Explicit Algebraic Number Theory.
    Lorentz Center, Leiden; September 27-October 2, 2002.
    Organizers: P. Stevenhagen, R. Tijdeman (Leiden).

Personal notes

  • New employees

    Permanent faculty:

    - Prof.dr. S.J. Edixhoven per September 1, 2002; the new professor in Geometry.
    Dear colleagues,
    it is nice for me to be back in the city where I was born, about 40 years ago. Together with my wife Reinie Erné and our two sons Luc and Tom (ages 7 and 4), we have settled in the Merenwijk in Leiden. During my absence from Leiden I have been in Rijswijk, Pijnacker, Utrecht (for studying mathematics, leading to a PhD in algebraic geometry), Berkeley (for two years, as an assistant professor), Utrecht (for one year, with a Huygens fellowship), Rennes (for ten years, as a professor). My main interests in mathematics are in arithmetic algebraic geometry. For more details please see my personal home page (still in Rennes): http://www.maths.univ-rennes1.fr/~edix.

    Veel groeten, Bas.

    - Prof.dr. R. van der Hout per September 1, 2002
    besides his LUF-professorship also a parttime appointment as senior researcher in the group Analysis (Prof.dr S. Verduyn Lunel).


    - Dr. M. Stupariu from July 1, 2002 - July 1, 2003, with Dr. M. Lübke
    M. Stupariu studied mathematics in Bucharest and received his PhD in 1998 from the University of Zurich. Afterwards he was assistant at the Universities of Bucharest and Zurich. In Leiden he will hold a post-doctoral position from July,1 2002 to July,1 2003 as a guest of Dr. M. Lubke. His research interests include topics in mathematical gauge theory and complex geometry.

    Ph.D. Students:

    - Drs. K.J. Batenburg per September 1, 2002, with Prof.dr R. Tijdeman and Dr.ir. H.J.J. te Riele (CWI).
    - Drs. T. van den Bogaart per September 1, 2002, with Prof.dr. S.J. Edixhoven.
    - Drs. B.H.J. Jansen per September 1, 2002, with Prof.dr. H.W. Lenstra.
    - Drs. G. Wiese per September 1, 2002, with Prof.dr. S.J. Edixhoven.

    Supporting staff:

    - R. van der Waal per June 1, 2002: education office and secretary managing board
    You can find me in room number 203. I am working as the secretary/coworker for the student's administration. I was born and raised in Leiden. For almost my whole life I have lived in Leiden. Last year I moved to Voorschoten, the place where I still live today. In my free time I sport a lot; my favorite sports are tennis and aerobic/steps. In the rest of my spare time I help my boyfriend (we are 7 years together) with the exposition of Chinese art. I have worked here now for a few months and I like it here because you are all nice people to work with.

    Greetings Rachel

    - Dr. J. Finkelnberg
    The position of Dr. J. Finkelnberg, at the moment responsible for the masterclasses and in-service training of teachers, will be enlarged per September 1, 2002. He will take care for the link-up between school (with its new tweede-fase systeem) and university. Per January 1, 2003 he will take over from C. Zaal the task Information and Public Relation.

  • Departed:

    - Prof.dr.ir. C. Roos per September 1, 2002 (now professor in Computer Science at TUD).
    - Dr. M. Fiocco per September 1, 2002 (to Department of Medicine - Prof.dr. J.C. van Houwelingen).
    - Dr. M. Girard per September 1, 2002.

  • Guests:

    Guests of Prof. S.A. van de Geer:

    - Prof. D. Donoho (Stanford University), Kloosterman Professor 2002, September 9-20 and November/December
    - Prof. Y. Rozenholc (Université Paris VII, INRA), September 1-December 31

    Ph.D. students of Prof. H.W. Lenstra from Berkeley:

    - A. Greicius, September 9-end of December
    - J. Voigt, September 15-November 15
    - J. Elliott, September 22-October 5 (perhaps one week longer)
    - R. van Luijk, until end of October


  • Doctoral exams, August 30, 2002

    - K.J. Batenburg
    Graduation code: RW1
    Master's Thesis: Analysis and optimization of an algorithm for discrete tomography
    Supervisor: Prof.dr. R. Tijdeman

    - M.O. de Kok
    Graduation code: C
    Master's Thesis: Isotopy extension and neighbourhood theorems in contact geometry
    Supervisor: Prof.dr. H. Geiges

    - N. Vink
    Graduation code: BW2
    Master's Thesis: Optimal premium rating by geographical area
    Supervisor: Prof.dr. S.A. van de Geer

    - P. Meijer
    Graduation code: RW1
    Master's Thesis: De krommen y3-y=axn en gewichtsverdelingen van irreducibele cyclische codes in karakteristiek 3
    Supervisor: Drs. M.A.J.G. van der Vlugt

    - J.J.M. van Winden
    Graduation code: RW1
    Master's Thesis: On Knaster's conjecture
    Supervisor: Prof.dr. H. Geiges

    - J. Ligtvoet
    Graduation code: BW2
    Master's Thesis: Democompass: Microsimulatie in discrete tijd
    Supervisor: Dr. E. Belitser

    - I. Grubisic
    Graduation code: RW2
    Master's Thesis: Interest rate theory: The BGM model
    Supervisor: Prof.dr. S.M. Verduyn Lunel

  • Propedeuse exams, August 30, 2002

    - A. Aaten
    - F.M. de Beer
    - P.J. Bruin
    - J.P. van der Horst
    - L. San Giorgi
    - A.P. Stolk
    - P.M.J. Tibboel
    - H.W. Verhoek
    - E.D. Werkhoven
    - G.G.A. Westhoff


  • Visits abroad:
    - G. van Dijk will visit the St. Petersburg university for a working visit from September 21-28 (on invitation)
    - M. Stupariu will attend the conference "Algebraic Geometry, Commutative Algebra and Topology" from September 22-26, Constanta (Romania).

  • Technical reports:
    MI 2002-13   Richard P. Groenewegen: Bounds for computing the tame kernel
    MI 2002-14   M. Girard: Group of Weierstrass points of a plane quartic with eight hyperflexes or more
    MI 2002-15   M. Girard: The group generated by the Weierstrass points of some plane quartics
    MI 2002-16   P. Meijer, M. van der Vlugt: The evaluation of Gauss sums for characters of 2-power order
    MI 2002-17   Roger D. Nussbaum, Sjoerd M. Verduyn Lunel: Asymptotic estimates for the periods of periodic points of nonexpansive maps
    MI 2002-18   Bernd Heidergott, Arie Hordijk, Heinz Weisshaupt: Measure-valued differentiation for stationary Markov chains
    MI 2002-19   Lambertus A. Peletier, José Antonio Rodríguez: Homoclinic orbits to a saddle-center in a fourth order differential equation
    MI 2002-20   Vivi Rottschäfer, Tasso J. Kaper: Geometric theory for multi-bump, self-similar, blowup solutions of the cubic nonlinear Schrödinger equation

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