Change detection in categorical evolving data streams

Detecting change in evolving data streams is a central issue for accurate adaptive learning. In real world applications, data streams have categorical features, and changes induced in the data distribution of these categorical features have not been considered extensively so far. Previous work on change detection focused on detecting changes in the accuracy of the learners, but without considering changes in the data distribution. To cope with these issues, we propose a new unsupervised change detection method, called CDCStream (Change Detection in Categorical Data Streams), well suited for categorical data streams. The proposed method is able to detect changes in a batch incremental scenario. It is based on the two following characteristics: (i) a summarization strategy is proposed to compress the actual batch by extracting a descriptive summary and (ii) a new segmentation algorithm is proposed to highlight changes and issue warnings for a data stream. To evaluate our proposal we employ it in a learning task over real world data and we compare its results with state of the art methods. We also report qualitative evaluation in order to show the behavior of CDCStream

Closed-loop control strategy with improved current for a flashing ratchet

We show how to switch on and off the ratchet potential of a collective Brownian motor, depending only on the position of the particles, in order to attain a current higher than or at least equal to that induced by any periodic flashing. Maximization of instant velocity turns out to be the optimal protocol for one particle but is nevertheless defeated by a periodic switching when a sufficiently large ensemble of particles is considered. The protocol presented in this article, although not the optimal one, yields approximately the same current as the optimal protocol for one particle and as the optimal periodic switching for an infinite number of them.Comment: 4 pages, 4 figure

A large accretion disk of extreme eccentricity in the TDE ASASSN-14li

In the canonical model for tidal disruption events (TDEs), the stellar debris circularizes quickly to form an accretion disk of size about twice the orbital pericenter of the star. Most TDEs and candidates discovered in the optical/UV have broad optical emission lines with complex and diverse profiles of puzzling origin. Liu et al. recently developed a relativistic elliptical disk model of constant eccentricity in radius for the broad optical emission lines of TDEs and well reproduced the double-peaked line profiles of the TDE candidate PTF09djl with a large and extremely eccentric accretion disk. In this paper, we show that the optical emission lines of the TDE ASASSN-14li with radically different profiles are well modelled with the relativistic elliptical disk model, too. The accretion disk of ASASSN-14li has an eccentricity 0.97 and semimajor axis of 847 times the Schwarzschild radius (r_S) of the black hole (BH). It forms as the consequence of tidal disruption of a star passing by a massive BH with orbital pericenter 25r_S. The optical emission lines of ASASSN-14li are powered by an extended X-ray source of flat radial distribution overlapping the bulk of the accretion disk and the single-peaked asymmetric line profiles are mainly due to the orbital motion of the emitting matter within the disk plane of inclination about 26\degr and of pericenter orientation closely toward the observer. Our results suggest that modelling the complex line profiles is powerful in probing the structures of accretion disks and coronal X-ray sources in TDEs.Comment: 10 pages, 8 figures, accepted for publication in the MNRA

Isotropic and Anisotropic Regimes of the Field-Dependent Spin Dynamics in Sr2IrO4: Raman Scattering Studies

A major focus of experimental interest in Sr2IrO4 has been to clarify how the magnetic excitations of this strongly spin-orbit coupled system differ from the predictions of anisotropic 2D spin-1/2 Heisenberg model and to explore the extent to which strong spin-orbit coupling affects the magnetic properties of iridates. Here, we present a high-resolution inelastic light (Raman) scattering study of the low energy magnetic excitation spectrum of Sr2IrO4 and doped Eu-doped Sr2IrO4 as functions of both temperature and applied magnetic field. We show that the high-field (H>1.5 T) in-plane spin dynamics of Sr2IrO4 are isotropic and governed by the interplay between the applied field and the small in-plane ferromagnetic spin components induced by the Dzyaloshinskii-Moriya interaction. However, the spin dynamics of Sr2IrO4 at lower fields (H<1.5 T) exhibit important effects associated with interlayer coupling and in-plane anisotropy, including a spin-flop transition at Hc in Sr2IrO4 that occurs either discontinuously or via a continuous rotation of the spins, depending upon the in-plane orientation of the applied field. These results show that in-plane anisotropy and interlayer coupling effects play important roles in the low-field magnetic and dynamical properties of Sr2IrO4.Comment: 8 pages, 4 figures, submitte

Evolution of Magnetism in Single-Crystal Honeycomb Iridates

We report the successful synthesis of single-crystals of the layered iridate, (Na$_{1-x}$Li$_{x}$)$_2$IrO$_3$, $0\leq x \leq 0.9$, and a thorough study of its structural, magnetic, thermal and transport properties. The new compound allows a controlled interpolation between Na$_2$IrO$_3$ and Li$_2$IrO$_3$, while maintaing the novel quantum magnetism of the honeycomb Ir$^{4+}$ planes. The measured phase diagram demonstrates a dramatic suppression of the N\'eel temperature, $T_N$, at intermediate $x$ suggesting that the magnetic order in Na$_2$IrO$_3$ and Li$_2$IrO$_3$ are distinct, and that at $x\approx 0.7$, the compound is close to a magnetically disordered phase that has been sought after in Na$_2$IrO$_3$ and Li$_2$IrO$_3$. By analyzing our magnetic data with a simple theoretical model we also show that the trigonal splitting, on the Ir$^{4+}$ ions changes sign from Na$_2$IrO$_3$ and Li$_2$IrO$_3$, and the honeycomb iridates are in the strong spin-orbit coupling regime, controlled by \jeff=1/2 moments.Comment: updated version with more dat

A low-bandgap dimeric porphyrin molecule for 10% efficiency solar cells with small photon energy loss

Dimeric porphyrin molecules have great potential as donor materials for high performance bulk heterojunction organic solar cells (OSCs). Recently reported dimeric porphyrins bridged by ethynylenes showed power conversion efficiencies (PCEs) of more than 8%. In this study, we design and synthesize a new conjugated dimeric D-A porphyrin ZnP2BT-RH, in which the two porphyrin units are linked by an electron accepting benzothiadiazole (BT) unit. The introduction of the BT unit enhances the electron delocalization, resulting in a lower highest occupied molecular orbital (HOMO) energy level and an increased molar extinction coefficient in the near-infrared (NIR) region. The bulk heterojunction solar cells with ZnP2BT-RH as the donor material exhibit a high PCE of up to 10% with a low energy loss (Eloss) of only 0.56 eV. The 10% PCE is the highest for porphyrin-based OSCs with a conventional structure, and this Eloss is also the smallest among those reported for small molecule-based OSCs with a PCE higher than 10% to date

How many radio-loud quasars can be detected by the Gamma-Ray Large Area Space Telescope?

In the unification scheme, radio quasars and FR II radio galaxies come from the same parent population, but viewed at different angles. Based on the Comptonization models for the gamma-ray emission from active galactic nuclei (AGNs), we estimate the number of radio quasars and FR II radio galaxies to be detected by the Gamma-Ray Large Area Space Telescope (GLAST) using the luminosity function (LF) of their parent population derived from the flat-spectrum radio quasar (FSRQ) LF. We find that ~1200 radio quasars will be detected by GLAST, if the soft seed photons for Comptonization come from the regions outside the jets. We also consider the synchrotron self-Comptonization (SSC) model, and find it unlikely to be responsible for gamma-ray emission from radio quasars. We find that no FR II radio galaxies will be detected by GLAST. Our results show that most radio AGNs to be detected by GLAST will be FSRQs (~99 % for the external Comptonization model, EC model), while the remainder (~1 %) will be steep-spectrum radio quasars (SSRQs). This implies that FSRQs will still be good candidates for identifying gamma-ray AGNs even for the GLAST sources. The contribution of all radio quasars and FR II radio galaxies to the extragalactic gamma-ray background (EGRB) is calculated, which accounts for ~30 % of the EGRB.Comment: 4 pages, accepted by ApJ Letter

Two-level matrix factorization for recommender systems

© 2015, The Natural Computing Applications Forum. Many existing recommendation methods such as matrix factorization (MF) mainly rely on user–item rating matrix, which sometimes is not informative enough, often suffering from the cold-start problem. To solve this challenge, complementary textual relations between items are incorporated into recommender systems (RS) in this paper. Specifically, we first apply a novel weighted textual matrix factorization (WTMF) approach to compute the semantic similarities between items, then integrate the inferred item semantic relations into MF and propose a two-level matrix factorization (TLMF) model for RS. Experimental results on two open data sets not only demonstrate the superiority of TLMF model over benchmark methods, but also show the effectiveness of TLMF for solving the cold-start problem