921 research outputs found
Optimizing minimal agents through abstraction
Abstraction is a valuable tool for dealing with scalability in large state space contexts. This paper addresses the design, using abstraction, of good policies for minimal autonomous agents applied within a situation-graph-framework. In this framework an agent’s policy is some function that maps perceptual inputs to actions deterministically. A good policy disposes the agent towards achieving one or more designated goal situations, and the design process aims to identify such policies. The agents to which the framework applies are assumed to have only partial observability, and in particular may not be able to perceive fully a goal situation. A further assumption is that the environment may influence an agent’s situation by unpredictable exogenous events, so that a policy cannot take advantage, of a reliable history of previous actions. The Bellman discount measure provides a means of evaluating situations and hence the overall value of a policy. When abstraction is used, the accuracy of the method can be significantly improved by modifying the standard Bellman equations. This paper describes the modification and demonstrates its power through comparison with simulation results
Gauge-Invariant Formulation of Spin-Current-Density Functional Theory
Spin-currents and non-abelian gauge potentials in electronic systems can be
treated by spin-current-density functional theory, whose main input is the
exchange-correlation (xc) energy expressed as a functional of spin-currents.
Constructing a functional of spin currents that is invariant under
U(1)SU(2) transformations is a long-standing challenge. We solve the
problem by expressing the energy as a functional of a new variable we call
"invariant vorticity". As an illustration we construct the xc energy functional
for a two-dimensional electron gas with linear spin-orbit coupling and show
that it is proportional to the fourth power of the spin current.Comment: 8 pages, 3 figures, submitte
Cross-shell excitation in two-proton knockout: Structure of Ca
The two-proton knockout reaction Be(Ti,Ca) has
been studied at 72 MeV/nucleon. Besides the strong feeding of the Ca
ground state, the only other sizeable cross section proceeds to a 3 level
at 3.9 MeV. There is no measurable direct yield to the first excited 2
state at 2.6 MeV. The results illustrate the potential of such direct reactions
for exploring cross-shell proton excitations in neutron-rich nuclei and
confirms the doubly-magic nature of Ca
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Connectionist modal logic: Representing modalities in neural networks
AbstractModal logics are amongst the most successful applied logical systems. Neural networks were proved to be effective learning systems. In this paper, we propose to combine the strengths of modal logics and neural networks by introducing Connectionist Modal Logics (CML). CML belongs to the domain of neural-symbolic integration, which concerns the application of problem-specific symbolic knowledge within the neurocomputing paradigm. In CML, one may represent, reason or learn modal logics using a neural network. This is achieved by a Modalities Algorithm that translates modal logic programs into neural network ensembles. We show that the translation is sound, i.e. the network ensemble computes a fixed-point meaning of the original modal program, acting as a distributed computational model for modal logic. We also show that the fixed-point computation terminates whenever the modal program is well-behaved. Finally, we validate CML as a computational model for integrated knowledge representation and learning by applying it to a well-known testbed for distributed knowledge representation. This paves the way for a range of applications on integrated knowledge representation and learning, from practical reasoning to evolving multi-agent systems
4-Dimensional BF Theory as a Topological Quantum Field Theory
Starting from a Lie group G whose Lie algebra is equipped with an invariant
nondegenerate symmetric bilinear form, we show that 4-dimensional BF theory
with cosmological term gives rise to a TQFT satisfying a generalization of
Atiyah's axioms to manifolds equipped with principal G-bundle. The case G =
GL(4,R) is especially interesting because every 4-manifold is then naturally
equipped with a principal G-bundle, namely its frame bundle. In this case, the
partition function of a compact oriented 4-manifold is the exponential of its
signature, and the resulting TQFT is isomorphic to that constructed by Crane
and Yetter using a state sum model, or by Broda using a surgery presentation of
4-manifolds.Comment: 15 pages in LaTe
Completeness and Incompleteness of Synchronous Kleene Algebra
Synchronous Kleene algebra (SKA), an extension of Kleene algebra (KA), was
proposed by Prisacariu as a tool for reasoning about programs that may execute
synchronously, i.e., in lock-step. We provide a countermodel witnessing that
the axioms of SKA are incomplete w.r.t. its language semantics, by exploiting a
lack of interaction between the synchronous product operator and the Kleene
star. We then propose an alternative set of axioms for SKA, based on Salomaa's
axiomatisation of regular languages, and show that these provide a sound and
complete characterisation w.r.t. the original language semantics.Comment: Accepted at MPC 201
Chern-Simons Field Theories with Non-semisimple Gauge Group of Symmetry
Subject of this work is a class of Chern-Simons field theories with
non-semisimple gauge group, which may well be considered as the most
straightforward generalization of an Abelian Chern-Simons field theory. As a
matter of fact these theories, which are characterized by a non-semisimple
group of gauge symmetry, have cubic interactions like those of non-abelian
Chern-Simons field theories, but are free from radiative corrections. Moreover,
at the tree level in the perturbative expansion,there are only two connected
tree diagrams, corresponding to the propagator and to the three vertex
originating from the cubic interaction terms. For such theories it is derived
here a set of BRST invariant observables, which lead to metric independent
amplitudes. The vacuum expectation values of these observables can be computed
exactly. From their expressions it is possible to isolate the Gauss linking
number and an invariant of the Milnor type, which describes the topological
relations among three or more closed curves.Comment: 16 pages, 1 figure, plain LaTeX + psfig.st
Nature of yrast excitations near N=40: Level structure of Ni-67
Excited states in Ni-67 were populated in deep-inelastic reactions of a Ni-64
beam at 430 MeV on a thick U-238 target. A level scheme built on the previously
known 13 micro-s isomer has been delineated up to an excitation energy of ~5.3
MeV and a tentative spin and parity of (21/2-). Shell model calculations have
been carried out using two effective interactions in the f5/2pg9/2 model space
with a Ni-56 core. Satisfactory agreement between experiment and theory is
achieved for the measured transition energies and branching ratios. The
calculations indicate that the yrast states are associated with rather complex
configurations, herewith demonstrating the relative weakness of the N=40
subshell gap and the importance of multi particle-hole excitations involving
the g9/2 neutron orbital.Comment: Accepted by Physical Review
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