Efficient Stochastic Approximation of Minimax Excess Risk Optimization

While traditional distributionally robust optimization (DRO) aims to minimize the maximal risk over a set of distributions, Agarwal and Zhang (2022) recently proposed a variant that replaces risk with excess risk. Compared to DRO, the new formulation -- minimax excess risk optimization (MERO) has the advantage of suppressing the effect of heterogeneous noise in different distributions. However, the choice of excess risk leads to a very challenging minimax optimization problem, and currently there exists only an inefficient algorithm for empirical MERO. In this paper, we develop efficient stochastic approximation approaches which directly target MERO. Specifically, we leverage techniques from stochastic convex optimization to estimate the minimal risk of every distribution, and solve MERO as a stochastic convex-concave optimization (SCCO) problem with biased gradients. The presence of bias makes existing theoretical guarantees of SCCO inapplicable, and fortunately, we demonstrate that the bias, caused by the estimation error of the minimal risk, is under-control. Thus, MERO can still be optimized with a nearly optimal convergence rate. Moreover, we investigate a practical scenario where the quantity of samples drawn from each distribution may differ, and propose a stochastic approach that delivers distribution-dependent convergence rates

Analysis and Research on Factors Affecting Information Dissemination of Emergencies in Social Media Environment

We examine whether emotional expression (joy, anger, sadness, fear, disgust) occurring in social media content is associated with a user’s information sharing behavior. Our research is in the context of Hurricane Irma and Tweets associated with that event. We find that negative emotions play an important role in the communication of information, among which, fear has the most significant effect. Meanwhile, the initial stages of the information life cycle have the most prominent influence on the information dissemination

Prognostic Value of Adrenomedullin and Natriuretic Peptides in Uroseptic Patients Induced by Ureteroscopy

The aim of this paper is to investigate whether urosepsis is related to irrigation pressure of ureteroscopy (URS) and evaluate the prognostic value of adrenomedullin (ADM) and atrial and brain natriuretic peptides (ANP and BNP) in URS-induced uroseptic patients. From July 2008 to October 2013, we enrolled 332 patients with untreated unilateral ureteral obstruction (UUO). The UUO group included three subgroups of, respectively, 118, 132, and 82 patients who underwent URS under intermittent stable irrigation pressure of, respectively, 80, 120, and 160 mmHg. The plasma concentrations of ADM, ANP, and BNP were measured in all subjects. URS was performed for all UUO patients; the values of the three peptides were measured again after URS. Irrigation pressure and stone size were independent risk factors of urosepsis. After URS, the plasma concentrations of ADM, ANP, and BNP were significantly higher in uroseptic patients. Moreover, the concentrations were significantly higher depending on the disease severity. Plasma concentrations of the three peptides were correlated with plasma ET concentration in the uroseptic patients. The areas under receiver operating characteristic (ROC) curve of ADM, ANP, and BNP for predicting urosepsis were 0.811, 0.728, and 0.764, respectively. In conclusion, ADM, along with ANP and BNP, is valuable for prognosis in urosepsis secondary to URS which is associated with irrigation pressure

Orbital Angular Momentum (OAM) Generation By Composite PEC-PMC Metasurfaces In Microwave Regime

Besides amplitude, phase and polarization, wave-front is also an important property of electromagnetic waves. EM waves with helical wavefront carrying orbital angular momentum (OAM) attract more attentions recently for its promising applications in communications. The OAM generation for microwaves was rarely reported. And existing designs are usually very complicated and suffer low efficiency problems. We propose composite PEC (perfect electric conductor)-PMC (perfect magnetic conductor) metasurfaces to convert the LCP (RCP) plane wave with zero OAM to the RCP (LCP) helical EM wave with desired OAM at microwave regime. The conversion efficiency can nearly reach 100%