12,293 research outputs found
Induced Ramsey-type results and binary predicates for point sets
Let and be positive integers and let be a finite point set in
general position in the plane. We say that is -Ramsey if there is a
finite point set such that for every -coloring of
there is a subset of such that and have the same order type
and is monochromatic in . Ne\v{s}et\v{r}il and Valtr proved
that for every , all point sets are -Ramsey. They also
proved that for every and , there are point sets that are
not -Ramsey.
As our main result, we introduce a new family of -Ramsey point sets,
extending a result of Ne\v{s}et\v{r}il and Valtr. We then use this new result
to show that for every there is a point set such that no function
that maps ordered pairs of distinct points from to a set of size
can satisfy the following "local consistency" property: if attains
the same values on two ordered triples of points from , then these triples
have the same orientation. Intuitively, this implies that there cannot be such
a function that is defined locally and determines the orientation of point
triples.Comment: 22 pages, 3 figures, final version, minor correction
Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution
We consider the formation of binary black hole mergers through the evolution
of field massive triple stars. In this scenario, favorable conditions for the
inspiral of a black hole binary are initiated by its gravitational interaction
with a distant companion, rather than by a common-envelope phase invoked in
standard binary evolution models. We use a code that follows self-consistently
the evolution of massive triple stars, combining the secular triple dynamics
(Lidov-Kozai cycles) with stellar evolution. After a black hole triple is
formed, its dynamical evolution is computed using either the orbit-averaged
equations of motion, or a high-precision direct integrator for triples with
weaker hierarchies for which the secular perturbation theory breaks down. Most
black hole mergers in our models are produced in the latter non-secular
dynamical regime. We derive the properties of the merging binaries and compute
a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to
~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random
orientation. Finally, we show that black hole mergers from the triple channel
have significantly higher eccentricities than those formed through the
evolution of massive binaries or in dense star clusters. Measured
eccentricities could therefore be used to uniquely identify binary mergers
formed through the evolution of triple stars. While our results suggest up to
~10 detections per year with Advanced-LIGO, the high eccentricities could
render the merging binaries harder to detect with planned space based
interferometers such as LISA.Comment: Accepted for publication in ApJ. 10 pages, 6 figure
Predictions for Triple Stars with and without a Pulsar in Star Clusters
Though about 80 pulsar binaries have been detected in globular clusters so
far, no pulsar has been found in a triple system in which all three objects are
of comparable mass. Here we present predictions for the abundance of such
triple systems, and for the most likely characteristics of these systems. Our
predictions are based on an extensive set of more than 500 direct simulations
of star clusters with primordial binaries, and a number of additional runs
containing primordial triples. Our simulations employ a number N_{tot} of equal
mass stars from N_{tot}=512 to N_{tot}=19661 and a primordial binary fraction
from 0-50%. In addition, we validate our results against simulations with
N=19661 that include a mass spectrum with a turn-off mass at 0.8 M_{sun},
appropriate to describe the old stellar populations of galactic globular
clusters. Based on our simulations, we expect that typical triple abundances in
the core of a dense cluster are two orders of magnitude lower than the binary
abundances, which in itself already suggests that we don't have to wait too
long for the first comparable-mass triple with a pulsar to be detected.Comment: 11 pages, minor changes to match MNRAS accepted versio
Majority voting on restricted domains
In judgment aggregation, unlike preference aggregation, not much is known about domain restrictions that guarantee consistent majority outcomes. We introduce several conditions on individual judgments sufficient for consistent majority judgments. Some are based on global orders of propositions or individuals, others on local orders, still others not on orders at all. Some generalize classic social-choice-theoretic domain conditions, others have no counterpart. Our most general condition generalizes Sen's triplewise value-restriction, itself the most general classic condition. We also prove a new characterization theorem: for a large class of domains, if there exists any aggregation function satisfying some democratic conditions, then majority voting is the unique such function. Taken together, our results support the robustness of majority rule
Algebraic foundations for qualitative calculi and networks
A qualitative representation is like an ordinary representation of a
relation algebra, but instead of requiring , as
we do for ordinary representations, we only require that , for each in the algebra. A constraint
network is qualitatively satisfiable if its nodes can be mapped to elements of
a qualitative representation, preserving the constraints. If a constraint
network is satisfiable then it is clearly qualitatively satisfiable, but the
converse can fail. However, for a wide range of relation algebras including the
point algebra, the Allen Interval Algebra, RCC8 and many others, a network is
satisfiable if and only if it is qualitatively satisfiable.
Unlike ordinary composition, the weak composition arising from qualitative
representations need not be associative, so we can generalise by considering
network satisfaction problems over non-associative algebras. We prove that
computationally, qualitative representations have many advantages over ordinary
representations: whereas many finite relation algebras have only infinite
representations, every finite qualitatively representable algebra has a finite
qualitative representation; the representability problem for (the atom
structures of) finite non-associative algebras is NP-complete; the network
satisfaction problem over a finite qualitatively representable algebra is
always in NP; the validity of equations over qualitative representations is
co-NP-complete. On the other hand we prove that there is no finite
axiomatisation of the class of qualitatively representable algebras.Comment: 22 page
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