Some formulations of this article have been discussed in the following blog dated June 10, 2006. http://www.dcorfield.pwp.blueyonder.co.uk/htdocs/2006_06_01_archive.html (I am endebted to Henning Krause for this reference).

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Claus Michael Ringel

Last modified: Tue Mar 13 14:34:59 CET 2007