Algebraic Combinatorics Seminar
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UNIVERSITY OF KENTUCKY COLLOQUIUM**

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The cd-index of fans, lattices and posets

Professor Kalle Karu

University of British Columbia

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Thursday, February 17, 2005

4 pm, Classroom Building 205

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Abstract:

Given an Eulerian poset P, for example, a polytope, a regular
CW-sphere, or a complete fan, one can encode the number of flags in it
(i.e., chains of faces of specified dimension) as the cd-index of P. In
this talk I will give a geometric interpretation of the cd-index,
different from the usual combinatorial construction. Using this
interpretation I will explain why the coefficients of the cd-index have
to be non-negative for complete fans, regular CW-spheres and, more
generally, for Gorenstein* posets.

Reference:
The cd-index of fans and lattices
,
Kalle Karu,
math.AG/0410513 (19 pages), 2004.