126,797 research outputs found
An Administrative View of Model Uncertainty in Public Health
Dr. Carrington reviews several ways to deal with model uncertainty, including those failing to acknowledge any use of models. He then evaluates six such methods with regard to, e.g., transparency and cost of execution
Reasoning about Minimal Belief and Negation as Failure
We investigate the problem of reasoning in the propositional fragment of
MBNF, the logic of minimal belief and negation as failure introduced by
Lifschitz, which can be considered as a unifying framework for several
nonmonotonic formalisms, including default logic, autoepistemic logic,
circumscription, epistemic queries, and logic programming. We characterize the
complexity and provide algorithms for reasoning in propositional MBNF. In
particular, we show that entailment in propositional MBNF lies at the third
level of the polynomial hierarchy, hence it is harder than reasoning in all the
above mentioned propositional formalisms for nonmonotonic reasoning. We also
prove the exact correspondence between negation as failure in MBNF and negative
introspection in Moore's autoepistemic logic
Probabilistic Default Reasoning with Conditional Constraints
We propose a combination of probabilistic reasoning from conditional
constraints with approaches to default reasoning from conditional knowledge
bases. In detail, we generalize the notions of Pearl's entailment in system Z,
Lehmann's lexicographic entailment, and Geffner's conditional entailment to
conditional constraints. We give some examples that show that the new notions
of z-, lexicographic, and conditional entailment have similar properties like
their classical counterparts. Moreover, we show that the new notions of z-,
lexicographic, and conditional entailment are proper generalizations of both
their classical counterparts and the classical notion of logical entailment for
conditional constraints.Comment: 8 pages; to appear in Proceedings of the Eighth International
Workshop on Nonmonotonic Reasoning, Special Session on Uncertainty Frameworks
in Nonmonotonic Reasoning, Breckenridge, Colorado, USA, 9-11 April 200
Polytropic dark halos of elliptical galaxies
The kinematics of stars and planetary nebulae in early type galaxies provide
vital clues to the enigmatic physics of their dark matter halos. We fit
published data for fourteen such galaxies using a spherical, self-gravitating
model with two components: (1) a Sersic stellar profile fixed according to
photometric parameters, and (2) a polytropic dark matter halo that conforms
consistently to the shared gravitational potential. The polytropic equation of
state can describe extended theories of dark matter involving self-interaction,
non-extensive thermostatistics, or boson condensation (in a classical limit).
In such models, the flat-cored mass profiles widely observed in disc galaxies
are due to innate dark physics, regardless of any baryonic agitation. One of
the natural parameters of this scenario is the number of effective thermal
degrees of freedom of dark matter (F_d) which is proportional to the dark heat
capacity. By default we assume a cosmic ratio of baryonic and dark mass.
Non-Sersic kinematic ideosyncrasies and possible non-sphericity thwart fitting
in some cases. In all fourteen galaxies the fit with a polytropic dark halo
improves or at least gives similar fits to the velocity dispersion profile,
compared to a stars-only model. The good halo fits usually prefer F_d values
from six to eight. This range complements the recently inferred limit of
7<F_d<10 (Saxton & Wu), derived from constraints on galaxy cluster core radii
and black hole masses. However a degeneracy remains: radial orbital anisotropy
or a depleted dark mass fraction could shift our models' preference towards
lower F_d; whereas a loss of baryons would favour higher F_d.Comment: 17 pages, 10 figures, 2 tables. MNRAS accepte
Designing Normative Theories for Ethical and Legal Reasoning: LogiKEy Framework, Methodology, and Tool Support
A framework and methodology---termed LogiKEy---for the design and engineering
of ethical reasoners, normative theories and deontic logics is presented. The
overall motivation is the development of suitable means for the control and
governance of intelligent autonomous systems. LogiKEy's unifying formal
framework is based on semantical embeddings of deontic logics, logic
combinations and ethico-legal domain theories in expressive classic
higher-order logic (HOL). This meta-logical approach enables the provision of
powerful tool support in LogiKEy: off-the-shelf theorem provers and model
finders for HOL are assisting the LogiKEy designer of ethical intelligent
agents to flexibly experiment with underlying logics and their combinations,
with ethico-legal domain theories, and with concrete examples---all at the same
time. Continuous improvements of these off-the-shelf provers, without further
ado, leverage the reasoning performance in LogiKEy. Case studies, in which the
LogiKEy framework and methodology has been applied and tested, give evidence
that HOL's undecidability often does not hinder efficient experimentation.Comment: 50 pages; 10 figure
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