12,714 research outputs found
The competition number of a graph and the dimension of its hole space
The competition graph of a digraph D is a (simple undirected) graph which has
the same vertex set as D and has an edge between x and y if and only if there
exists a vertex v in D such that (x,v) and (y,v) are arcs of D. For any graph
G, G together with sufficiently many isolated vertices is the competition graph
of some acyclic digraph. The competition number k(G) of G is the smallest
number of such isolated vertices. In general, it is hard to compute the
competition number k(G) for a graph G and it has been one of important research
problems in the study of competition graphs to characterize a graph by its
competition number. Recently, the relationship between the competition number
and the number of holes of a graph is being studied. A hole of a graph is a
cycle of length at least 4 as an induced subgraph. In this paper, we conjecture
that the dimension of the hole space of a graph is no smaller than the
competition number of the graph. We verify this conjecture for various kinds of
graphs and show that our conjectured inequality is indeed an equality for
connected triangle-free graphs.Comment: 6 pages, 3 figure
The competition numbers of ternary Hamming graphs
It is known to be a hard problem to compute the competition number k(G) of a
graph G in general. Park and Sano [13] gave the exact values of the competition
numbers of Hamming graphs H(n,q) if or . In
this paper, we give an explicit formula of the competition numbers of ternary
Hamming graphs.Comment: 6 pages, 2 figure
Structure and Problem Hardness: Goal Asymmetry and DPLL Proofs in<br> SAT-Based Planning
In Verification and in (optimal) AI Planning, a successful method is to
formulate the application as boolean satisfiability (SAT), and solve it with
state-of-the-art DPLL-based procedures. There is a lack of understanding of why
this works so well. Focussing on the Planning context, we identify a form of
problem structure concerned with the symmetrical or asymmetrical nature of the
cost of achieving the individual planning goals. We quantify this sort of
structure with a simple numeric parameter called AsymRatio, ranging between 0
and 1. We run experiments in 10 benchmark domains from the International
Planning Competitions since 2000; we show that AsymRatio is a good indicator of
SAT solver performance in 8 of these domains. We then examine carefully crafted
synthetic planning domains that allow control of the amount of structure, and
that are clean enough for a rigorous analysis of the combinatorial search
space. The domains are parameterized by size, and by the amount of structure.
The CNFs we examine are unsatisfiable, encoding one planning step less than the
length of the optimal plan. We prove upper and lower bounds on the size of the
best possible DPLL refutations, under different settings of the amount of
structure, as a function of size. We also identify the best possible sets of
branching variables (backdoors). With minimum AsymRatio, we prove exponential
lower bounds, and identify minimal backdoors of size linear in the number of
variables. With maximum AsymRatio, we identify logarithmic DPLL refutations
(and backdoors), showing a doubly exponential gap between the two structural
extreme cases. The reasons for this behavior -- the proof arguments --
illuminate the prototypical patterns of structure causing the empirical
behavior observed in the competition benchmarks
Fragile Black Holes
The AdS/CFT correspondence may give a new way of understanding field theories
in extreme conditions, as in the quark-gluon plasma phase of quark matter. The
correspondence normally involves asymptotically AdS black holes with dual field
theories which are defined on locally flat boundary spacetimes; the implicit
assumption is that the distortions of spacetime which occur under extreme
conditions do not affect the field theory in any unexpected way. However, AdS
black holes are [to varying degrees] "fragile", in the sense that they become
unstable to stringy effects when their event horizons are sufficiently
distorted. This implies that field theories on curved backgrounds may likewise
be unstable in a suitable sense. We investigate this phenomenon, focussing on
the "fragility" of AdS black holes with flat event horizons. We find that, when
they are distorted, these black holes are always unstable in string theory.
This may have consequences for the detailed structure of the quark matter phase
diagram at extreme values of the spacetime curvature.Comment: 24 pages, 5 figures; reference added; to appear in Nuclear Physics
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