Rotting infinitely many-armed bandits

We consider the infinitely many-armed bandit problem with rotting rewards, where the mean reward of an arm decreases at each pull of the arm according to an arbitrary trend with maximum rotting rate ϱ=o(1). We show that this learning problem has an Ω(max{ϱ1/3T,T−−√}) worst-case regret lower bound where T is the time horizon. We show that a matching upper bound O~(max{ϱ1/3T,T−−√}), up to a poly-logarithmic factor, can be achieved by an algorithm that uses a UCB index for each arm and a threshold value to decide whether to continue pulling an arm or remove the arm from further consideration, when the algorithm knows the value of the maximum rotting rate ϱ. We also show that an O~(max{ϱ1/3T,T3/4}) regret upper bound can be achieved by an algorithm that does not know the value of ϱ, by using an adaptive UCB index along with an adaptive threshold value

Contextual Linear Bandits under Noisy Features: Towards Bayesian Oracles

We study contextual linear bandit problems under uncertainty on features; they are noisy with missing entries. To address the challenges from the noise, we analyze Bayesian oracles given observed noisy features. Our Bayesian analysis finds that the optimal hypothesis can be far from the underlying realizability function, depending on noise characteristics, which is highly non-intuitive and does not occur for classical noiseless setups. This implies that classical approaches cannot guarantee a non-trivial regret bound. We thus propose an algorithm aiming at the Bayesian oracle from observed information under this model, achieving $\tilde{O}(d\sqrt{T})$ regret bound with respect to feature dimension $d$ and time horizon $T$. We demonstrate the proposed algorithm using synthetic and real-world datasets.Comment: 30 page

An Atypical Mitral Valve Prolapse in a Patient With Behçet's Disease

We report the case of a 42-year-old male who was admitted to the hospital with progressive dyspnea. Cardiomegaly and diffuse pulmonary edema were visible on chest X-ray and multiple oral and genital ulcers on physical examination. On admission, echocardiography revealed mitral valve prolapse (MVP) predominantly involving a basal portion of the posterior leaflet, with severe mitral regurgitation. A successful mitral valve replacement with St. Jude #29 was performed, after pre-treatment with prednisolone for 2 weeks. Fifteen months following the operation, the patient expired from severe pulmonary edema and secondary pneumonia. This case demonstrates, for the first time in the literature, an unusual feature of mitral prolapse in the basal portion with severe mitral regurgitation in a patient with Behçet's disease. As suggested by this case, we should consider an atypical type of MVP as a possible inflammatory involvement of the heart in patients with Behçet's disease

Emergence of robust 2D skyrmions in SrRuO3 ultrathin film without the capping layer

Magnetic skyrmions have fast evolved from a novelty, as a realization of topologically protected structure with particle-like character, into a promising platform for new types of magnetic storage. Significant engineering progress was achieved with the synthesis of compounds hosting room-temperature skyrmions in magnetic heterostructures, with the interfacial Dzyaloshinskii-Moriya interactions (DMI) conducive to the skyrmion formation. Here we report findings of ultrathin skyrmion formation in a few layers of SrRuO3 grown on SrTiO3 substrate without the heavy-metal capping layer. Measurement of the topological Hall effect (THE) reveals a robust stability of skyrmions in this platform, judging from the high value of the critical field 1.57 Tesla (T) at low temperature. THE survives as the field is tilted by as much as 85 degrees at 10 Kelvin, with the in-plane magnetic field reaching up to 6.5 T. Coherent Bragg Rod Analysis, or COBRA for short, on the same film proves the rumpling of the Ru-O plane to be the source of inversion symmetry breaking and DMI. First-principles calculations based on the structure obtained from COBRA find significant magnetic anisotropy in the SrRuO3 film to be the main source of skyrmion robustness. These features promise a few-layer SRO to be an important new platform for skyrmionics, without the necessity of introducing the capping layer to boost the spin-orbit coupling strength artificially.Comment: Supplementary Information available upon reques

Emergence of robust 2D skyrmions in SrRuO3 ultrathin film without the capping layer

Magnetic skyrmions have fast evolved from a novelty, as a realization of topologically protected structure with particle-like character, into a promising platform for new types of magnetic storage. Significant engineering progress was achieved with the synthesis of compounds hosting room-temperature skyrmions in magnetic heterostructures, with the interfacial Dzyaloshinskii-Moriya interactions (DMI) conducive to the skyrmion formation. Here we report findings of ultrathin skyrmion formation in a few layers of SrRuO3 grown on SrTiO3 substrate without the heavy-metal capping layer. Measurement of the topological Hall effect (THE) reveals a robust stability of skyrmions in this platform, judging from the high value of the critical field 1.57 Tesla (T) at low temperature. THE survives as the field is tilted by as much as 85 degrees at 10 Kelvin, with the in-plane magnetic field reaching up to 6.5 T. Coherent Bragg Rod Analysis, or COBRA for short, on the same film proves the rumpling of the Ru-O plane to be the source of inversion symmetry breaking and DMI. First-principles calculations based on the structure obtained from COBRA find significant magnetic anisotropy in the SrRuO3 film to be the main source of skyrmion robustness. These features promise a few-layer SRO to be an important new platform for skyrmionics, without the necessity of introducing the capping layer to boost the spin-orbit coupling strength artificially.Comment: Supplementary Information available upon reques

The Risk of Type 2 Diabetes Mellitus according to Changes in Obesity Status in Late Middle-Aged Adults: A Nationwide Cohort Study of Korea

Background Although obesity is a well-known risk factor of type 2 diabetes mellitus (T2DM), there is scant data on discriminating the contribution of previous obesity and recent weight gain on developing T2DM. Methods We analyzed the Korean National Health Insurance Service-Health Screening Cohort data from 2002 to 2015 where Korean residents underwent biennial health checkups. Participants were classified into four groups according to their obesity status (body mass index [BMI] ≥25 kg/m2) before and after turning 50 years old: maintaining normal (MN), becoming obese (BO), becoming normal (BN), and maintaining obese (MO). Cox proportional hazards regression model was used to estimate the risk of T2DM factoring in the covariates age, sex, BMI, presence of impaired fasting glucose or hypertension, family history of diabetes, and smoking status. Results A total of 118,438 participants (mean age, 52.5±1.1 years; men, 45.2%) were prospectively evaluated for incident T2DM. A total of 7,339 (6.2%) participants were diagnosed with T2DM during a follow-up period of 4.8±2.6 years. Incidence rates of T2DM per 1,000 person-year were 9.20 in MN, 14.81 in BO, 14.42 in BN, 21.38 in MO. After factoring in covariates, participants in the groups BN (adjusted hazard ratio [aHR], 1.15; 95% confidence interval [CI], 1.04 to 1.27) and MO (aHR, 1.14; 95% CI, 1.06 to 1.24) were at increased risk of developing T2DM compared to MN, whereas BO (hazard ratio, 1.06; 95% CI, 0.96 to 1.17) was not. Conclusion Having been obese before 50 years old increased the risk of developing T2DM in the future, but becoming obese after 50 did not. Therefore, it is important to maintain normal weight from early adulthood to prevent future metabolic perturbations

A New Hardware Correlator in Korea: Performance Evaluation using KVN observations

We report results of the performance evaluation of a new hardware correlator in Korea, the Daejeon correlator, developed by the Korea Astronomy and Space Science Institute (KASI) and the National Astronomical Observatory of Japan (NAOJ). We conducted Very Long Baseline Interferometry (VLBI) observations at 22~GHz with the Korean VLBI Network (KVN) in Korea and the VLBI Exploration of Radio Astrometry (VERA) in Japan, and correlated the aquired data with the Daejeon correlator. For evaluating the performance of the new hardware correlator, we compared the correlation outputs from the Daejeon correlator for KVN observations with those from a software correlator, the Distributed FX (DiFX). We investigated the correlated flux densities and brightness distributions of extragalactic compact radio sources. The comparison of the two correlator outputs show that they are consistent with each other within $<8\%$, which is comparable with the amplitude calibration uncertainties of KVN observations at 22~GHz. We also found that the 8\% difference in flux density is caused mainly by (a) the difference in the way of fringe phase tracking between the DiFX software correlator and the Daejeon hardware correlator, and (b) an unusual pattern (a double-layer pattern) of the amplitude correlation output from the Daejeon correlator. The visibility amplitude loss by the double-layer pattern is as small as 3\%. We conclude that the new hardware correlator produces reasonable correlation outputs for continuum observations, which are consistent with the outputs from the DiFX software correlator.Comment: 13 pagee, 9 figures, 3 tables, to appear in JKAS (received February 9, 2015; accepted March 16, 2015