43 research outputs found
Counting inequivalent monotone Boolean functions
Monotone Boolean functions (MBFs) are Boolean functions satisfying the monotonicity condition for any . The number of MBFs in n variables is
known as the th Dedekind number. It is a longstanding computational
challenge to determine these numbers exactly - these values are only known for
at most 8. Two monotone Boolean functions are inequivalent if one can be
obtained from the other by renaming the variables. The number of inequivalent
MBFs in variables was known only for up to . In this paper we
propose a strategy to count inequivalent MBF's by breaking the calculation into
parts based on the profiles of these functions. As a result we are able to
compute the number of inequivalent MBFs in 7 variables. The number obtained is
490013148
Embedding a pair of graphs in a surface, and the width of 4-dimensional prismatoids
A prismatoid is a polytope with all its vertices contained in two parallel
facets, called its bases. Its width is the number of steps needed to go from
one base to the other in the dual graph. The first author recently showed that
the existence of counter-examples to the Hirsch conjecture is equivalent to
that of -prismatoids of width larger than , and constructed such
prismatoids in dimension five. Here we show that the same is impossible in
dimension four. This is proved by looking at the pair of graph embeddings on a
2-sphere that arise from the normal fans of the two bases.Comment: This paper merges and supersedes the papers arXiv:1101.3050 (of the
last two authors) and arXiv:1102.2645 (of the first author
Colourful Simplicial Depth
Inspired by Barany's colourful Caratheodory theorem, we introduce a colourful
generalization of Liu's simplicial depth. We prove a parity property and
conjecture that the minimum colourful simplicial depth of any core point in any
d-dimensional configuration is d^2+1 and that the maximum is d^(d+1)+1. We
exhibit configurations attaining each of these depths and apply our results to
the problem of bounding monochrome (non-colourful) simplicial depth.Comment: 18 pages, 5 figues. Minor polishin
Expected Crossing Numbers
The expected value for the weighted crossing number of a randomly weighted
graph is studied. A variation of the Crossing Lemma for expectations is proved.
We focus on the case where the edge-weights are independent random variables
that are uniformly distributed on [0,1].Comment: 14 page
Universal State Transfer on Graphs
A continuous-time quantum walk on a graph is given by the unitary matrix
, where is the Hermitian adjacency matrix of . We say
has pretty good state transfer between vertices and if for any
, there is a time , where the -entry of
satisfies . This notion was introduced by Godsil
(2011). The state transfer is perfect if the above holds for . In
this work, we study a natural extension of this notion called universal state
transfer. Here, state transfer exists between every pair of vertices of the
graph. We prove the following results about graphs with this stronger property:
(1) Graphs with universal state transfer have distinct eigenvalues and flat
eigenbasis (where each eigenvector has entries which are equal in magnitude).
(2) The switching automorphism group of a graph with universal state transfer
is abelian and its order divides the size of the graph. Moreover, if the state
transfer is perfect, then the switching automorphism group is cyclic. (3) There
is a family of prime-length cycles with complex weights which has universal
pretty good state transfer. This provides a concrete example of an infinite
family of graphs with the universal property. (4) There exists a class of
graphs with real symmetric adjacency matrices which has universal pretty good
state transfer. In contrast, Kay (2011) proved that no graph with real-valued
adjacency matrix can have universal perfect state transfer. We also provide a
spectral characterization of universal perfect state transfer graphs that are
switching equivalent to circulants.Comment: 27 pages, 3 figure