Nearly optimal solutions for the Chow Parameters Problem and low-weight approximation of halfspaces

The \emph{Chow parameters} of a Boolean function $f: \{-1,1\}^n \to \{-1,1\}$ are its $n+1$ degree-0 and degree-1 Fourier coefficients. It has been known since 1961 (Chow, Tannenbaum) that the (exact values of the) Chow parameters of any linear threshold function $f$ uniquely specify $f$ within the space of all Boolean functions, but until recently (O'Donnell and Servedio) nothing was known about efficient algorithms for \emph{reconstructing} $f$ (exactly or approximately) from exact or approximate values of its Chow parameters. We refer to this reconstruction problem as the \emph{Chow Parameters Problem.} Our main result is a new algorithm for the Chow Parameters Problem which, given (sufficiently accurate approximations to) the Chow parameters of any linear threshold function $f$, runs in time \tilde{O}(n^2)\cdot (1/\eps)^{O(\log^2(1/\eps))} and with high probability outputs a representation of an LTF $f'$ that is \eps-close to $f$. The only previous algorithm (O'Donnell and Servedio) had running time \poly(n) \cdot 2^{2^{\tilde{O}(1/\eps^2)}}. As a byproduct of our approach, we show that for any linear threshold function $f$ over $\{-1,1\}^n$, there is a linear threshold function $f'$ which is \eps-close to $f$ and has all weights that are integers at most \sqrt{n} \cdot (1/\eps)^{O(\log^2(1/\eps))}. This significantly improves the best previous result of Diakonikolas and Servedio which gave a \poly(n) \cdot 2^{\tilde{O}(1/\eps^{2/3})} weight bound, and is close to the known lower bound of $\max\{\sqrt{n},$ (1/\eps)^{\Omega(\log \log (1/\eps))}\} (Goldberg, Servedio). Our techniques also yield improved algorithms for related problems in learning theory

Dust-penetrated morphology in the high-redshift universe: clues from NGC 922

Results from the Hubble Deep Field (HDF) North and South show a large percentage of high-redshift galaxies whose appearance falls outside traditional classification systems. The nature of these objects is poorly understood, but sub-mm observations indicate that at least some of these systems are heavily obscured (Sanders 2000). This raises the intriguing possibility that a physically meaningful classification system for high-redshift galaxies might be more easily devised at rest-frame infrared wavelengths, rather than in the optical regime. Practical realization of this idea will become possible with the advent of the Next Generation Space Telescope (NGST). In order to explore the capability of NGST for undertaking such science, we present NASA-IRTF and SCUBA observations of NGC 922, a chaotic system in our local Universe which bears a striking resemblance to objects such as HDF 2-86 (z=0.749) in the HDF North. If objects such as NGC 922 are common at high-redshifts, then this galaxy may serve as a local morphological `Rosetta stone' bridging low and high-redshift populations. In this paper we demonstrate that quantitative measures of galactic structure are recoverable in the rest-frame infrared for NGC 922 seen at high redshifts using NGST, by simulating the appearance of this galaxy at redshifts z=0.7 and z=1.2 in rest-frame K'. Our results suggest that the capability of efficiently exploring the rest-wavelength IR morphology of high-z galaxies should probably be a key factor in deciding the final choice of instruments for the NGST.Comment: 7 pages, 12 Figures. Accepted for publication in A&A. Better version of the figures can be found at http://www.inaoep.mx/~puerari/ngs

Avaliação da capacidade de adesão de células de biofilmes de Candida após tratamento com nanopartículas de prata

O objetivo deste estudo foi investigar a capacidade de adesão a células epiteliais humanas e a superfície de poliestireno de leveduras viáveis recuperadas de biofilmes de Candida albicans e Candida glabrata tratados com nanopartículas de prata (NP). Métodos: Biofilmes de Candida (48 hrs) foram formados em placas de microtitulação de 6 poços e tratados por 24 horas com NP (5 nm) nas concentrações de 13,5 e 54 mg/L. Suspensões de células de Candida (107 células viáveis/mL em RPMI 1640) provenientes dos biofilmes tratados com NP foram adicionadas a monocamadas de células HeLa e a poços vazios de placas de microtitulação de 24 poços (para estudar adesão a poliestireno). Após 2 horas de contato, a adesão das leveduras foi determinada usando a coloração com violeta cristal. Resultados: A capacidade de adesão de leveduras viáveis a células HeLa e a superfícies de poliestireno foi significativamente reduzida, e esta redução foi maior quando os biofilmes foram pré-tratados com NP na concentração de 54 mg/L. Ainda, a quantidade de leveduras aderidas das duas cepas diferiu de acordo com o substrato (células epiteliais e superfície de poliestireno). Conclusão: NP podem induzir modificações em leveduras viáveis, as quais podem diminuir a disseminação de infecções por Candida, principalmente em pacientes imunocomprometidos

The ACS Virgo Cluster Survey XV. The Formation Efficiencies of Globular Clusters in Early-Type Galaxies: The Effects of Mass and Environment

The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey, the largest homogeneous catalog of its kind. We find that 1) GC mass fractions can be high in both giants and dwarfs, but are universally low in galaxies with intermediate luminosities. 2) The behavior of specific frequency across galaxy mass is dominated by the blue GCs. 3) The GC fractions of low-mass galaxies exhibit a dependence on environment. Nearly all dwarf galaxies with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarfs is biased toward dense environments. 4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to the Millennium Simulation show that central dwarfs have older stellar populations and form more stars at higher star formation rates (SFRs) and SFR surface densities. The SFR surface density in simulated dwarfs peaks before the total SFR, naturally producing GC populations that are older and more metal-poor than the field stars. 5) Dwarfs within ~40 kpc of the giant ellipticals M87 and M49 are red and have few or no GCs, suggesting that they have been tidally stripped and have contributed their GCs to the halos of their giant neighbors. The central dwarfs with high GC mass fractions are thus likely to be the survivors most similar to the protogalaxies that assembled the rich M87 globular cluster system.(Abridged)Comment: 27 pages, 21 figures, 7 tables. Accepted for publication in the Astrophysical Journa

Nanopartículas de prata : análise dos efeitos anti-biofilme e anti-adesão sobre Candida albicans e Candida glabrata

Os objetivos deste trabalho foram: (1) avaliar o efeito sinergístico de nanopartículas de prata (NP) com Nistatina e Clorexidina contra biofilmes de Candida albicans e Candida glabrata; (2) verificar o efeito das NP na composição da matriz destes biofilmes e (3) investigar a capacidade de adesão a células epiteliais HeLa e a poliestireno de leveduras tratadas com NP. As drogas sozinhas ou em combinação com NP (5 nm) foram aplicadas sobre biofilmes maduros (48 h) e após 24 h de contato a atividade sinergística foi avaliada através da quantificação da biomassa total e por meio da contagem do número de colônias. Após o tratamento com NP, as matrizes dos biofilmes foram extraídas e analisadas em termos de proteínas, carboidratos e DNA. Ainda, leveduras viáveis foram recuperadas e adicionadas tanto às células HeLa quanto aos poços vazios de placas de poliestireno e, após 2 horas de contato, a adesão foi determinada usando violeta cristal. NP combinadas com Nistatina e Clorexidina exibiram atividade anti-biofilme sinergística dependente das espécies e concentrações de drogas usadas. Ainda, NP interferiram na composição da matriz extracelular dos biofilmes e a capacidade de adesão das leveduras viáveis foi significativamente reduzida após tratamento prévio com NP. Esses achados permitem concluir que NP podem contribuir na prevenção ou tratamento da estomatite protética associada à Candida. Entretanto, estudos adicionais são necessários para que estas NP sejam usadas com segurança

The DEEP Groth Strip Galaxy Redshift Survey. III. Redshift Catalog and Properties of Galaxies

The Deep Extragalactic Evolutionary Probe (DEEP) is a series of spectroscopic surveys of faint galaxies, targeted at the properties and clustering of galaxies at redshifts z ~ 1. We present the redshift catalog of the DEEP 1 GSS pilot phase of this project, a Keck/LRIS survey in the HST/WFPC2 Groth Survey Strip. The redshift catalog and data, including reduced spectra, are publicly available through a Web-accessible database. The catalog contains 658 secure galaxy redshifts with a median z=0.65, and shows large-scale structure walls to z = 1. We find a bimodal distribution in the galaxy color-magnitude diagram which persists to z = 1. A similar color division has been seen locally by the SDSS and to z ~ 1 by COMBO-17. For red galaxies, we find a reddening of only 0.11 mag from z ~ 0.8 to now, about half the color evolution measured by COMBO-17. We measure structural properties of the galaxies from the HST imaging, and find that the color division corresponds generally to a structural division. Most red galaxies, ~ 75%, are centrally concentrated, with a red bulge or spheroid, while blue galaxies usually have exponential profiles. However, there are two subclasses of red galaxies that are not bulge-dominated: edge-on disks and a second category which we term diffuse red galaxies (DIFRGs). The distant edge-on disks are similar in appearance and frequency to those at low redshift, but analogs of DIFRGs are rare among local red galaxies. DIFRGs have significant emission lines, indicating that they are reddened mainly by dust rather than age. The DIFRGs in our sample are all at z>0.64, suggesting that DIFRGs are more prevalent at high redshifts; they may be related to the dusty or irregular extremely red objects (EROs) beyond z>1.2 that have been found in deep K-selected surveys. (abridged)Comment: ApJ in press. 24 pages, 17 figures (12 color). The DEEP public database is available at http://saci.ucolick.org

Análise do efeito de nanopartículas de prata contra células aderidas e biofilmes de Candida albicans e Candida glabrata

O aumento na resistência dos biofilmes de Candida à terapia antifúngica convencional tem despertado o interesse no uso da prata como um agente antimicrobiano. Assim, o objetivo deste trabalho foi avaliar a eficácia antifúngica de nanopartículas de prata (NPs) contra células aderidas e biofilmes de Candida albicans e Candida glabrata. Métodos: NPs esféricas (5 nm) foram sintetizadas através da redução do nitrato de prata pelo citrato de sódio. Testes de mínima concentração inibitória (MCI) foram realizados para as duas espécies de Candida de acordo com o método da microdiluição. NPs foram aplicadas sobre células aderidas (2 hrs) e biofilmes (48 hrs), e após 24 horas de contato os biofilmes resultantes foram caracterizados através da contagem do número de unidades formadoras de colônias (UFCs) e quantificação da biomassa total. Resultados: Os valores de MCI para C. glabrata foram maiores (0,4 – 3,3 µg/mL) do que para C. albicans (0,4 – 1,6 µg/mL). NPs foram mais efetivas na redução da biomassa total quando aplicadas sobre células aderidas do que sobre biofilmes pré-formados. NPs também foram altamente efetivas na redução das UFCs quando aplicadas sobre as células aderidas de C. glabrata (~70%) e respectivos biofilmes (~50%). Para as cepas de C. albicans o efeito não foi tão notório, mas também existiu uma redução no número de UFCs. Conclusão: NPs apresentam potencial como agente antifúngico alternativo no controle de infecções por espécies de Candida

Antifungal activity of different silver nanoparticles suspensions against Candida biofilms

Objective: The tolerance of Candida biofilms to conventional antifungal drugs has stimulated the search for new therapies that could prevent or treat Candida-associated denture stomatitis. The objectives of this study were (i) to assess the antibiofilm activity of different silver nanoparticles (SN) suspensions against Candida albicans and Candida glabrata biofilms and (ii) to evaluate the effect of these nanoparticles on the matrix composition and the structure of Candida biofilms. Method: SN of three different average sizes (5, 10 and 60 nm) were used in this study. Biofilms of an oral clinical isolate of C. albicans and C. glabrata were formed for 48 h. These biofilms were exposed to different concentrations of SN for a period of 24 h. Antibiofilm activity of SN was determined by total biomass quantification (using crystal violet staining) and colony forming units enumeration. The extracellular matrices of Candida biofilms were extracted and analyzed chemically in terms of proteins, carbohydrates and DNA. Scanning electron microscopy (SEM) and epifluorescence microscopy were used to analyze the structure and morphology of the biofilms. Result: SN were effective in reducing total biomass (around 90%) of C. glabrata biofilms. All SN suspensions showed significant reduction of the number of cultivable cells at concentrations equal to or higher than 108 µg/mL. The particle size and the type of stabilizing agent did not interfere in the antibiofilm activity of SN. Interestingly, SN induce modifications in the contents of protein, carbohydrate and DNA of the extracellular matrices of Candida biofilms. SEM and epifluorescence investigations revealed structural differences in spatial arrangement and damage in the cell walls of biofilm cells, compared to the controls. Conclusion: SN present therapeutic potential and might be useful in the treatment of denture stomatitis