5,173 research outputs found
Lifshitz Holography with Isotropic Scale Invariance
Is it possible for an anisotropic Lifshitz critical point to actually exhibit
isotropic conformal invariance? We answer this question in the affirmative by
constructing a concrete holographic realization. We study three-dimensional
spin-3 higher-spin gauge theory with a z=2 Lifshitz ground state with
non-trivial spin-3 background. We provide consistent boundary conditions and
determine the associated asymptotic symmetry algebra. Surprisingly, we find
that the algebra consists of two copies of the W_3 extended conformal algebra,
which is the extended conformal algebra of an isotropic critical system.
Moreover, the central charges are given by 3l/(2G). We consider the possible
geometric interpretation of the theory in light of the higher spin gauge
invariance and remark on the implications of the asymptotic symmetry analysis.Comment: 19 pp; v2: minor edits, new refs; v3: added footnote, minor
rewordings, to appear in JHE
Calibration of Distributionally Robust Empirical Optimization Models
We study the out-of-sample properties of robust empirical optimization
problems with smooth -divergence penalties and smooth concave objective
functions, and develop a theory for data-driven calibration of the non-negative
"robustness parameter" that controls the size of the deviations from
the nominal model. Building on the intuition that robust optimization reduces
the sensitivity of the expected reward to errors in the model by controlling
the spread of the reward distribution, we show that the first-order benefit of
``little bit of robustness" (i.e., small, positive) is a significant
reduction in the variance of the out-of-sample reward while the corresponding
impact on the mean is almost an order of magnitude smaller. One implication is
that substantial variance (sensitivity) reduction is possible at little cost if
the robustness parameter is properly calibrated. To this end, we introduce the
notion of a robust mean-variance frontier to select the robustness parameter
and show that it can be approximated using resampling methods like the
bootstrap. Our examples show that robust solutions resulting from "open loop"
calibration methods (e.g., selecting a confidence level regardless of
the data and objective function) can be very conservative out-of-sample, while
those corresponding to the robustness parameter that optimizes an estimate of
the out-of-sample expected reward (e.g., via the bootstrap) with no regard for
the variance are often insufficiently robust.Comment: 51 page
Evaluation of User Support: Factors That Affect User Satisfaction With Helpdesks and Helplines
In addition to technical documentation, face-to-face helpdesks and telephonic helplines are a powerful means for supporting users of technical products and services. This study investigates the factors that determine user satisfaction with helpdesks and helplines. A survey, based on the SERVQUAL framework and questionnaire, shows that the SERVQUAL dimensions of customer satisfaction are not applicable in these contexts. Three quality dimensions were found instead: solution quality, the experience of the consultation, and, in the case of a physical environment, the so-called tangibles. Helpdesk customers base their overall quality perceptions mainly on their experiences during a consultation, while helpline customers focus strongly on the quality of the solution offered.\ud
The study also found a connection between the perceived helpline quality and the appreciation of the primary service
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