Below the content is listed in the reverse chronological order of the first addition or last substantial modification of an item. 

Homepage of the paper 
Homepage of the ongoing research 
Homepage of the paper 
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at distinguished chair of Pacific Institute of Mathematical Sciencies in Calgary in FebruaryMarch 2000 
streaming video from Calgary, Canada 
at colloquium of Department of Mathematics and Statistique of the University of Calgary on March 9, 2000 
streaming video from Calgary, Canada 
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at a seminar of IREM in Paris 
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An invited lecture on this subject was presented at JAF'13 and was published in the Proceedings, ISBN 2951153902, 119134 (1997). 
A version of this paper was submitted to the Electronic Journal of Combinatorics but did not meet the high standards of this Journal. 
A version of this paper appeared in Theoretical Computer Science , 257(12):167183, 2001. 
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Published in Acta Arithmetica, XCI.4, 1999, 367378 
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Preliminary version of full paper: .ps 
Originally published in The Mathematical Intelligencer, vol.14, no.4, 1992, pp.3845 
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homepage of the book 
.html (last modified on April 7, 1998) 
.tex .dvi .ps 
.tex .dvi .ps 
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Abstract and BibTeX entry  .dvi  .ps.Z 
abstract,  .ps (9 pages) 