87,625 research outputs found
Relating two Hopf algebras built from an operad
Starting from an operad, one can build a family of posets. From this family
of posets, one can define an incidence Hopf algebra. By another construction,
one can also build a group directly from the operad. We then consider its Hopf
algebra of functions. We prove that there exists a surjective morphism from the
latter Hopf algebra to the former one. This is illustrated by the case of an
operad built on rooted trees, the \NAP operad, where the incidence Hopf
algebra is identified with the Connes-Kreimer Hopf algebra of rooted trees.Comment: 21 pages, use graphics, 12 figures Version 2 : references added,
minor changes. This version has not been corrected after submission. The
final and corrected version will appear in IMRN and can be obtained from the
author
Censorship challenges to books in Scottish public libraries
Censorship challenges to books in UK public libraries have received renewed attention recently. This study sought to establish the incidence of censorship challenges to books in Scottish public libraries in the years 2005-2009 and the actions taken in response to these challenges. It was found that eight local authorities in Scotland had received formal censorship challenges to books, with a total of 15 challenges throughout the country. The most common action taken in response to these challenges was for the book to be kept in stock in its original position with the rationale for this explained to the complainer, with the second most common action being taken to move the title to another section of the library. Two books were removed from the library in response to a censorship challenge. The largest numbers of challenges were made against books on the basis of sexual material
Simplicial and Cellular Trees
Much information about a graph can be obtained by studying its spanning
trees. On the other hand, a graph can be regarded as a 1-dimensional cell
complex, raising the question of developing a theory of trees in higher
dimension. As observed first by Bolker, Kalai and Adin, and more recently by
numerous authors, the fundamental topological properties of a tree --- namely
acyclicity and connectedness --- can be generalized to arbitrary dimension as
the vanishing of certain cellular homology groups. This point of view is
consistent with the matroid-theoretic approach to graphs, and yields
higher-dimensional analogues of classical enumerative results including
Cayley's formula and the matrix-tree theorem. A subtlety of the
higher-dimensional case is that enumeration must account for the possibility of
torsion homology in trees, which is always trivial for graphs. Cellular trees
are the starting point for further high-dimensional extensions of concepts from
algebraic graph theory including the critical group, cut and flow spaces, and
discrete dynamical systems such as the abelian sandpile model.Comment: 39 pages (including 5-page bibliography); 5 figures. Chapter for
forthcoming IMA volume "Recent Trends in Combinatorics
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