16,988 research outputs found
Exact solutions of embedding the four-dimensional perfect fluid in a five- or higher-dimensional Einstein spacetime and the cosmological interpretations
We investigate an exact solution that describes the embedding of the
four-dimensional (4D) perfect fluid in a five-dimensional (5D) Einstein
spacetime. The effective metric of the 4D perfect fluid as a hypersurface with
induced matter is equivalent to the Robertson-Walker metric of cosmology. This
general solution shows interconnections among many 5D solutions, such as the
solution in the braneworld scenario and the topological black hole with
cosmological constant. If the 5D cosmological constant is positive, the metric
periodically depends on the extra dimension. Thus we can compactify the extra
dimension on and study the phenomenological issues. We also generalize
the metric ansatz to the higher-dimensional case, in which the 4D part of the
Einstein equations can be reduced to a linear equation.Comment: 8 pages, 1 figures; v2: minor errors corrected; v3: references added;
v4: matches the version to appear in PL
Learning to Ask Conversational Questions by Optimizing Levenshtein Distance
Conversational Question Simplification (CQS) aims to simplify self-contained
questions into conversational ones by incorporating some conversational
characteristics, e.g., anaphora and ellipsis. Existing maximum likelihood
estimation (MLE) based methods often get trapped in easily learned tokens as
all tokens are treated equally during training. In this work, we introduce a
Reinforcement Iterative Sequence Editing (RISE) framework that optimizes the
minimum Levenshtein distance (MLD) through explicit editing actions. RISE is
able to pay attention to tokens that are related to conversational
characteristics. To train RISE, we devise an Iterative Reinforce Training (IRT)
algorithm with a Dynamic Programming based Sampling (DPS) process to improve
exploration. Experimental results on two benchmark datasets show that RISE
significantly outperforms state-of-the-art methods and generalizes well on
unseen data.Comment: 13 pages, 4 figures, Published in ACL 202
Neutron scattering study of magnetic phase separation in nanocrystalline LaCaMnO
We demonstrate that magnetic phase separation and competing spin order in the
colossal magnetoresistive (CMR) manganites can be directly explored via tuning
strain in bulk samples of nanocrystalline LaCaMnO. Our results
show that strain can be reversibly frozen into the lattice in order to
stabilize coexisting antiferromagnetic domains within the nominally
ferromagnetic metallic state of LaCaMnO. The measurement of
tunable phase separation via magnetic neutron powder diffraction presents a
direct route of exploring the correlated spin properties of phase separated
charge/magnetic order in highly strained CMR materials and opens a potential
avenue for realizing intergrain spin tunnel junction networks with enhanced CMR
behavior in a chemically homogeneous material.Comment: 6 pages, 4 figures. New figure and text added to manuscrip
Collective iteration behavior for online social networks
Understanding the patterns of collective behavior in online social network (OSNs) is critical to expanding the knowledge of human behavior and tie relationship. In this paper, we investigate a specific pattern called social signature in Facebook and Wiki users’ online communication behaviors, capturing the distribution of frequency of interactions between different alters over time in the ego network. The empirical results show that there are robust social signatures of interactions no matter how friends change over time, which indicates that a stable commutation pattern exists in online communication. By comparing a random null model, we find the that commutation pattern is heterogeneous between ego and alters. Furthermore, in order to regenerate the pattern of the social signature, we present a preferential interaction model, which assumes that new users intend to look for the old users with strong ties while old users have tendency to interact with new friends. The experimental results show that the presented model can reproduce the heterogeneity of social signature by adjusting 2 parameters, the number of communicating targets m and the max number of interactions n, for Facebook users, m=n=5, for Wiki users, m=2 and n=8. This work helps in deeply understanding the regularity of social signature
A Neutral Two Flavor Loff Color Superconductor
In this paper we construct analytically a LOFF color superconducting state
that is both color and charge neutral using the weak coupling approximation. We
demonstrate that this state is free from chromomagnetic instabilities. Its
relevance to the realistic quark matter at moderately high baryon density is
discussed.Comment: 9 pages, Latex, correction of algebraic mistake and typo
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