Algebraic Combinatorics Seminar


The cd-index of fans, lattices and posets

Professor Kalle Karu
University of British Columbia

Thursday, February 17, 2005
4 pm, Classroom Building 205


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.