67 research outputs found
A Moment-Matching Approach to Testable Learning and a New Characterization of Rademacher Complexity
A remarkable recent paper by Rubinfeld and Vasilyan (2022) initiated the
study of \emph{testable learning}, where the goal is to replace hard-to-verify
distributional assumptions (such as Gaussianity) with efficiently testable ones
and to require that the learner succeed whenever the unknown distribution
passes the corresponding test. In this model, they gave an efficient algorithm
for learning halfspaces under testable assumptions that are provably satisfied
by Gaussians.
In this paper we give a powerful new approach for developing algorithms for
testable learning using tools from moment matching and metric distances in
probability. We obtain efficient testable learners for any concept class that
admits low-degree \emph{sandwiching polynomials}, capturing most important
examples for which we have ordinary agnostic learners. We recover the results
of Rubinfeld and Vasilyan as a corollary of our techniques while achieving
improved, near-optimal sample complexity bounds for a broad range of concept
classes and distributions.
Surprisingly, we show that the information-theoretic sample complexity of
testable learning is tightly characterized by the Rademacher complexity of the
concept class, one of the most well-studied measures in statistical learning
theory. In particular, uniform convergence is necessary and sufficient for
testable learning. This leads to a fundamental separation from (ordinary)
distribution-specific agnostic learning, where uniform convergence is
sufficient but not necessary.Comment: 34 page
CFD simulations on the effect of catalysts on the hydrodeoxygenation of bio-oil
Bio-oil derived from lignocellulose biomass is an emerging alternative resource to conventional fossil fuel. However, the as-obtained unprocessed bio oil is oxy-rich, has low pH and contains high moisture, which suppresses the heating value; thus, its mixing with conventional fuel is not compatible. Therefore, studies on the upgradation of bio oil using catalytic hydrodeoxygenation (HDO) have become prominent in recent years. This study presents computational fluid dynamics (CFD) based simulation results on the effect of catalysts (Pt/Al2O3, Ni–Mo/Al2O3, Co–Mo/Al2O3) on the upgradation of bio oil using a hydrodeoxygenation process in an ebullated bed reactor. These numerical simulations are performed using an Eulerian multiphase flow module that is available in a commercial CFD based solver, ANSYS Fluent 14.5. Prior to obtaining the new results, the present numerical solution methodology is validated by reproducing some of the experimental results on the upgradation of bio oil available in the literature. Furthermore, the influence of weight hourly space velocities (WHSVs), operating temperature, and pressure inside the reactor for the different catalysts on the performance of HDO for bio oil upgradation in an ebullated bed reactor are delineated. It is observed that the gaseous stream products are higher in the presence of Pt/Al2O3 catalyst; phenols are higher when Ni–Mo/Al2O3 is used, and higher aromatics are obtained with the Co–Mo/Al2O3 catalyst. Finally, a comparison among the mass fraction of the individual species of three phases with respect to different catalysts for various combinations of WHSV, temperature and pressure values are presented
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