26 research outputs found
Accelerated Quasi-Newton Proximal Extragradient: Faster Rate for Smooth Convex Optimization
In this paper, we propose an accelerated quasi-Newton proximal extragradient
(A-QPNE) method for solving unconstrained smooth convex optimization problems.
With access only to the gradients of the objective, we prove that our method
can achieve a convergence rate of , where is the problem dimension and
is the number of iterations. In particular, in the regime where , our method matches the optimal rate of by
Nesterov's accelerated gradient (NAG). Moreover, in the the regime where , it outperforms NAG and converges at a faster rate of
. To the best of our knowledge,
this result is the first to demonstrate a provable gain of a quasi-Newton-type
method over NAG in the convex setting. To achieve such results, we build our
method on a recent variant of the Monteiro-Svaiter acceleration framework and
adopt an online learning perspective to update the Hessian approximation
matrices, in which we relate the convergence rate of our method to the dynamic
regret of a specific online convex optimization problem in the space of
matrices.Comment: 44 pages, 1 figur
Online Learning Guided Curvature Approximation: A Quasi-Newton Method with Global Non-Asymptotic Superlinear Convergence
Quasi-Newton algorithms are among the most popular iterative methods for
solving unconstrained minimization problems, largely due to their favorable
superlinear convergence property. However, existing results for these
algorithms are limited as they provide either (i) a global convergence
guarantee with an asymptotic superlinear convergence rate, or (ii) a local
non-asymptotic superlinear rate for the case that the initial point and the
initial Hessian approximation are chosen properly. In particular, no current
analysis for quasi-Newton methods guarantees global convergence with an
explicit superlinear convergence rate. In this paper, we close this gap and
present the first globally convergent quasi-Newton method with an explicit
non-asymptotic superlinear convergence rate. Unlike classical quasi-Newton
methods, we build our algorithm upon the hybrid proximal extragradient method
and propose a novel online learning framework for updating the Hessian
approximation matrices. Specifically, guided by the convergence analysis, we
formulate the Hessian approximation update as an online convex optimization
problem in the space of matrices, and we relate the bounded regret of the
online problem to the superlinear convergence of our method.Comment: 33 pages, 1 figure, accepted to COLT 202
Projection-Free Methods for Stochastic Simple Bilevel Optimization with Convex Lower-level Problem
In this paper, we study a class of stochastic bilevel optimization problems,
also known as stochastic simple bilevel optimization, where we minimize a
smooth stochastic objective function over the optimal solution set of another
stochastic convex optimization problem. We introduce novel stochastic bilevel
optimization methods that locally approximate the solution set of the
lower-level problem via a stochastic cutting plane, and then run a conditional
gradient update with variance reduction techniques to control the error induced
by using stochastic gradients. For the case that the upper-level function is
convex, our method requires
stochastic
oracle queries to obtain a solution that is -optimal for the
upper-level and -optimal for the lower-level. This guarantee
improves the previous best-known complexity of
. Moreover, for the
case that the upper-level function is non-convex, our method requires at most
stochastic
oracle queries to find an -stationary point. In the
finite-sum setting, we show that the number of stochastic oracle calls required
by our method are and
for the convex and non-convex
settings, respectively, where
Future Gradient Descent for Adapting the Temporal Shifting Data Distribution in Online Recommendation Systems
One of the key challenges of learning an online recommendation model is the
temporal domain shift, which causes the mismatch between the training and
testing data distribution and hence domain generalization error. To overcome,
we propose to learn a meta future gradient generator that forecasts the
gradient information of the future data distribution for training so that the
recommendation model can be trained as if we were able to look ahead at the
future of its deployment. Compared with Batch Update, a widely used paradigm,
our theory suggests that the proposed algorithm achieves smaller temporal
domain generalization error measured by a gradient variation term in a local
regret. We demonstrate the empirical advantage by comparing with various
representative baselines
The predictive value of preoperative luteinizing hormone to follicle stimulating hormone ratio for ovulation abnormalities recovery after laparoscopic sleeve gastrectomy: A prospective cohort study
IntroductionObesity-related ovulation abnormalities (OA) affect fertility. LSG is the most frequent bariatric operation. However, no research has identified a reliable indicator for predicting OA recovery after LSG. The purpose of this research was to examine the prognostic usefulness of preoperative the luteinizing hormone (LH) to follicle-stimulating hormone (FSH) ratio (LFR).MethodsOur department conducted a prospective study from 2016 to 2021. Venous blood was typically tested 3 days before surgery to get the preoperative LFR. Descriptive data, preoperative and postoperative variables were also collected. Binary logistic regression related preoperative LFR with OA recovery. The receiver operating characteristic (ROC) curve evulated preoperative LFR’s predictive capability.ResultsA total of 157 women with a complete follow-up of one year were included. LFR was the only factor linked with OA (P < 0.001). AUC (area under the ROC curve) = 0.915, cutoff = 1.782, sensitivity = 0.93, and specificity = 0.82.DiscussionOverall, LSG has a favorable surgical result, with a %TWL of 66.082 ± 12.012 at 12 months postoperatively. Preoperative sexual hormone levels, as expressed by LFR, has the potential to predict the fate of OA following LSG at one year post-operatively
Towards high-quality biodiesel production from microalgae using original and anaerobically-digested livestock wastewater
In this study, we conducted proof-of-concept research towards the simultaneous treatment of livestock wastewater and the generation of high-quality biodiesel, through microalgae technology. Both original (OPE) and anaerobically-digested (DPE) piggery effluents were investigated for the culture of the microalgae, Desmodesmus sp. EJ8-10. After 14 days’ cultivation, the dry biomass from microalgae cultivated in OPE increased from an initial value of 0.01 g/L to 0.33-0.39 g/L, while those growing in DPE only achieved a final dried mass of 0.15-0.35 g/L, under similar initial ammonium nitrogen (NH4+-N) concentrations. The significantly higher microalgal biomass production achieved in the OPE medium may have been supported by the abundance of both macronutrient, such as phosphorus (P), and of micronutrients, such as trace elements, present in the OPE, which may not been present in similar quantities in the DPE. However, a higher lipid content was observed (19.4-28%) in microalgal cells from DPE cultures than those (18.7-22.3%) from OPE cultures. Moreover, the fatty acid compositions in the microalgae cultured in DPE contained high levels of monounsaturated fatty acids (MUFAs) and total C16-C18 acids, which would afford a superior potential for high-quality biodiesel production. The N/P ratio (15.4:1) in OPE was much closer to that indicated by previous studies to be the most suitable (16:1) for microalgae growth, when compared with that determined from the DPE culture medium. This may facilitate protein synthesis in the algal cells and induce a lower accumulation of lipids. Based on these findings, we proposed a new flowsheet for sustainable livestock waste managemen
Valorisation of microalgae residues after lipid extraction: Pyrolysis characteristics for biofuel production
As a promising source of renewable energy, biofuel from microalgae pyrolysis is seen as a competitive alternative to fossil fuels. However, currently, the widely applied pre-treatment process of lipid extraction results in large amounts of microalgae residues, which though with energy potential, being considered as process wastes and ignored of its re-utilization potential. In this study, a new workflow of biofuel generation from microalgae biomass through lipid extraction and pyrolysis of defatted microalgae residues was proposed and assessed. The effects of lipid extraction and pyrolysis temperature (350–750 ℃) on pyrolysis products were investigated, and pyrolysis pathways were postulated. To address the twin goals of lowering emission of pollutants and elevating energy products, an optimal pyrolysis temperature of 650 ℃ was suggested. After extraction of lipids, the relative contents of valuable products (aromatic, aliphatic hydrocarbons and fatty acids) and some harmful by-products, e.g., PAHs, significantly reduced, while other harmful substrates, e.g., nitrogen-compounds increased. Mechanistic investigations indicated that pyrolysis of proteins without the presence of lipids could promote higher production of nitrogen-containing organics and aromatics. These results reveal the effects of lipid extraction and variation of temperature on microalgal pyrolysis, and also provide a basis for full utilization of microalgae as an aid to alleviate many fossil energy problems