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MIT Open Access Articles

Mostrando recursos 1 - 20 de 20.822

  1. The global future stability of the FLRW solutions to the Dust-Einstein system with a positive cosmological constant

    Hadžić, Mahir; Speck, Jared R.
    We study small perturbations of the Friedman–Lemaître–Robertson–Walker (FLRW) solutions to the dust-Einstein system with a positive cosmological constant in the case that the space-like Cauchy hypersurfaces are diffeomorphic to 3. We show that the FLRW solutions are nonlinearly globally future-stable under small perturbations of their initial data. In our analysis, we construct harmonic-type coordinates such that the cosmological constant results in the presence of dissipative terms in the evolution equations. Our result extends those of [I. Rodnianski and J. Speck, The nonlinear future stability of the FLRW family of solutions to the irrotational Euler–Einstein system with a positive cosmological constant,...

  2. Quantification and Genetic Analysis of Salivirus/Klassevirus in Wastewater in Arizona, USA

    Kitajima, Masaaki; Haramoto, Eiji; Iker, Brandon C.; Rachmadi, Andri T.; Gerba, Charles P.
    Salivirus/klassevirus sequences were identified in 7 (15 %) wastewater samples collected in Arizona monthly for a year, with the highest concentration of 2.28 × 105 and 2.46 × 104 copies/L in influent and effluent, respectively. This is the first report of quantification and genetic analysis of salivirus/klassevirus in water samples in the United States.

  3. A new route to silyl-substituted cyclobutenones and silylketenes

    Benda, Konstantin; Knoth, Tanja; Schaumann, Ernst; Danheiser, Rick Lane
    2-Silyl-cyclobutene(di)ones are obtained by an addition/substitution approach on dimethyl squarate using silyl anions. The acetal and in particular the thioacetal derivatives readily undergo electrocyclic ring opening to reactive silyl(vinyl)ketenes.

  4. Optical cavity for improved performance of solar receivers in solar-thermal systems

    Weinstein, Lee Adragon; Kraemer, Daniel; McEnaney, Kenneth; Chen, Gang
    A principal loss mechanism for solar receivers in solar-thermal systems is radiation from the absorbing surface. This loss can be reduced by using the concept of directional selectivity in which radiation is suppressed at angles larger than the incident angle of the sunlight striking the absorber. Directional selectivity can achieve efficiencies similar to high solar concentration, without the drawbacks associated with large heat fluxes. A specularly reflective hemispherical cavity placed over the absorber can reflect emitted radiation back to the absorber, effectively suppressing emission losses. An aperture in the cavity will still allow sunlight to reach the absorber surface when...

  5. Sparse sums of squares on finite abelian groups and improved semidefinite lifts

    Fawzi, Hamza; Saunderson, James F; Parrilo, Pablo A
    Let G be a finite abelian group. This paper is concerned with nonnegative functions on G that are sparse with respect to the Fourier basis. We establish combinatorial conditions on subsets S and T of Fourier basis elements under which nonnegative functions with Fourier support S are sums of squares of functions with Fourier support T. Our combinatorial condition involves constructing a chordal cover of a graph related to G and S (the Cayley graph Cay(G^,S)) with maximal cliques related to T. Our result relies on two main ingredients: the decomposition of sparse positive semidefinite matrices with a chordal sparsity...

  6. Tonic zinc inhibits spontaneous firing in dorsal cochlear nucleus principal neurons by enhancing glycinergic neurotransmission

    Perez-Rosello, Tamara; Anderson, Charles T.; Ling, Cindy; Lippard, Stephen J.; Tzounopoulos, Thanos
    In many synapses of the CNS, mobile zinc is packaged into glutamatergic vesicles and co-released with glutamate during neurotransmission. Following synaptic release, the mobilized zinc modulates ligand- and voltage-gated channels and receptors, functioning as an inhibitory neuromodulator. However, the origin and role of tonic, as opposed to phasically released, zinc are less well understood. We investigated tonic zinc in the dorsal cochlear nucleus (DCN), a zinc-rich, auditory brainstem nucleus. Our results show that application of a high-affinity, extracellular zinc chelator (ZX1) enhances spontaneous firing in DCN principal neurons (fusiform cells), consistent with inhibition of this neuronal property by tonic zinc....

  7. Exploiting ontology graph for predicting sparsely annotated gene function

    Wang, Sheng; Zhai, ChengXiang; Peng, Jian; Cho, Hyunghoon; Berger Leighton, Bonnie
    Motivation: Systematically predicting gene (or protein) function based on molecular interaction networks has become an important tool in refining and enhancing the existing annotation catalogs, such as the Gene Ontology (GO) database. However, functional labels with only a few (<10) annotated genes, which constitute about half of the GO terms in yeast, mouse and human, pose a unique challenge in that any prediction algorithm that independently considers each label faces a paucity of information and thus is prone to capture non-generalizable patterns in the data, resulting in poor predictive performance. There exist a variety of algorithms for function prediction, but...

  8. Active Particles with Soft and Curved Walls: Equation of State, Ratchets, and Instabilities

    Nikola, Nikolai; Kafri, Yariv; Tailleur, Julien; Voituriez, Raphaël; Solon, Alexandre; Kardar, Mehran
    We study, from first principles, the pressure exerted by an active fluid of spherical particles on general boundaries in two dimensions. We show that, despite the nonuniform pressure along curved walls, an equation of state is recovered upon a proper spatial averaging. This holds even in the presence of pairwise interactions between particles or when asymmetric walls induce ratchet currents, which are accompanied by spontaneous shear stresses on the walls. For flexible obstacles, the pressure inhomogeneities lead to a modulational instability as well as to the spontaneous motion of short semiflexible filaments. Finally, we relate the force exerted on objects...

  9. Diagnosis of physical and biological control over phytoplankton in the Gulf of Maine-Georges Bank region using an adjoint data assimilation approach

    Wang, Caixia; Rizzoli, Paola M
    The linkage between physical and biological processes, particularly the effect of the circulation field on the distribution of phytoplankton, is studied by applying a two-dimensional model and an adjoint data assimilation approach to the Gulf of Maine-Georges Bank region. The model results, comparing well with observation data, reveal seasonal and geographic variations of phytoplankton concentration and verify that the seasonal cycles of phytoplankton are controlled by both biological sources and advection processes which are functions of space and time and counterbalance each other. Although advective flux divergences have greater magnitudes on Georges Bank than in the coastal region of the...

  10. Diagnosis of physical and biological controls on phytoplankton distribution in the Sargasso Sea

    Wang, Caixia; Rizzoli, Paola M
    The linkage between physical and biological processes is studied by applying a one-dimensional physical-biological coupled model to the Sargasso Sea. The physical model is the Princeton Ocean Model and the biological model is a five-component system including phytoplankton, zooplankton, nitrate, ammonium, and detritus. The coupling between the physical and biological model is accomplished through vertical mixing which is parameterized by the level 2.5 Mellor and Yamada turbulence closure scheme. The coupled model investigates the annual cycle of ecosystem production and the response to external forcing, such as heat flux, wind stress, and surface salinity, and the relative importance of physical...

  11. Methods to Estimate the Comparative Effectiveness of Clinical Strategies that Administer the Same Intervention at Different Times

    Huitfeldt, Anders; Kalager, Mette; Hoff, Geir; Robins, James M.; Hernan, Miguel Angel
    Clinical guidelines that rely on observational data due to the absence of data from randomized trials benefit when the observational data or its analysis emulates trial data or its analysis. In this paper, we review a methodology for emulating trials that compare the effects of different timing strategies, that is, strategies that vary the frequency of delivery of a medical intervention or procedure. We review trial emulation for comparing (i) single applications of the procedure at different times, (ii) fixed schedules of application, and (iii) schedules adapted to the evolving clinical characteristics of the patients. For illustration, we describe an...

  12. Microbial taxonomy in the post-genomic era: Rebuilding from scratch?

    Campeão, Mariana; Sawabe, Tomoo; Swings, Jean; Thompson, Cristiane C.; Amaral, Gilda R.; Edwards, Robert A.; Polz, Martin F.; Dutilh, Bas E.; Ussery, David W.; Thompson, Fabiano L.; Polz, Martin F
    Microbial taxonomy should provide adequate descriptions of bacterial, archaeal, and eukaryotic microbial diversity in ecological, clinical, and industrial environments. Its cornerstone, the prokaryote species has been re-evaluated twice. It is time to revisit polyphasic taxonomy, its principles, and its practice, including its underlying pragmatic species concept. Ultimately, we will be able to realize an old dream of our predecessor taxonomists and build a genomic-based microbial taxonomy, using standardized and automated curation of high-quality complete genome sequences as the new gold standard.

  13. Loss of Tet Enzymes Compromises Proper Differentiation of Embryonic Stem Cells

    Dawlaty, Meelad M.; Breiling, Achim; Le, Thuc; Barrasa, M. Inmaculada; Raddatz, Günter; Gao, Qing; Powell, Benjamin E.; Cheng, Albert W.; Faull, Kym F.; Lyko, Frank; Jaenisch, Rudolf
    Tet enzymes (Tet1/2/3) convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and are dynamically expressed during development. Whereas loss of individual Tet enzymes or combined deficiency of Tet1/2 allows for embryogenesis, the effect of complete loss of Tet activity and 5hmC marks in development is not established. We have generated Tet1/2/3 triple-knockout (TKO) mouse embryonic stem cells (ESCs) and examined their developmental potential. Combined deficiency of all three Tets depleted 5hmC and impaired ESC differentiation, as seen in poorly differentiated TKO embryoid bodies (EBs) and teratomas. Consistent with impaired differentiation, TKO ESCs contributed poorly to chimeric embryos, a defect rescued by Tet1...

  14. Enzyme-Catalyzed Macrocyclization of Long Unprotected Peptides

    Zhang, Chi; Dai, Peng; Spokoyny, Alexander M.; Pentelute, Bradley L.
    A glutathione S-transferase (GST) catalyzed macrocyclization reaction for peptides up to 40 amino acids in length is reported. GST catalyzes the selective SNAr reaction between an N-terminal glutathione (GSH, γ-Glu-Cys-Gly) tag and a C-terminal perfluoroaryl-modified cysteine on the same polypeptide chain. Cyclic peptides ranging from 9 to 24 residues were quantitatively produced within 2 h in aqueous pH = 8 buffer at room temperature. The reaction was highly selective for cyclization at the GSH tag, enabling the combination of GST-catalyzed ligation with native chemical ligation to generate a large 40-residue peptide macrocycle.

  15. Oscillations in light-triggered logic microfluidic circuit

    Cartas Ayala, Marco Aurelio; Gilson, Laura M.; Shen, Chong; Karnik, Rohit
    Control of droplets in microfluidic environments has numerous applications ranging from analysis and sample preparation for biomaterials synthesis (Mann and Ozin Nature 382:313–318, 1996) and medical diagnostics (Pipper et al. Nat Med 13:1259–1263, 2007) to photonics (Schmidt and Hawkins Nat Photonics 5:598–604, 2011). Here we study the oscillations present in a microfluidic circuit capable of sorting curable droplets on demand by triggering the circuit with UV-light. Prior to this paper we showed that a simple circuit can self-sort particles and produce a binary output, sorted or rejected stream of particles, based on the hydrodynamic resistance induced by the particles as...

  16. Genetically engineered mouse models of cancer reveal new insights about the antitumor immune response

    DuPage, Michel J.; Jacks, Tyler E
    Cancer is a complex disease that can originate in virtually all the tissues of the body, and tumors progress through many different stages during their development. While genetic mutations in the emerging cancer cells drive this disease, it has become increasingly clear that cancer development is strongly influenced by the surrounding microenvironment. Cells of the immune system are critical components of this extrinsic network of cancer regulators, contributing significantly to the microenvironment of most cancers and either promoting or inhibiting the initiation and progression of this disease. Genetically engineered mouse (GEM) mouse models of spontaneous cancer are starting to shape...

  17. Distributed Torque Control of Deloaded Wind DFIGs for Wind Farm Power Output Regulation

    Baros, Stefanos; Ilić, Marija D.
    In this work, we propose a distributed control methodology that solves the problem of dispatching and regulating the power outputs of a group of deloaded wind Double- Fed Induction Generators (DFIGs) so that wind farm (WF) power output regulation is attained. Our control methodology guarantees additionally that fair load-sharing among the DFIGs is achieved. In our context, fair load-sharing means that, the ratio defined by the mechanical power over the maximum power from the wind i.e the utilization level, of all DFIGs, is the same in steady state. Dynamic, distributed dispatch and control of the power outputs of DFIGs refers...

  18. Distributed Consensus Control of DFIGs with Storage for Wind Farm Power Output Regulation

    Baros, Stefanos; Ilić, Marija D.
    Today the state-of-the-art (SoA) wind generators (WGs) are the double-fed induction (DFIGs) with integrated storage devices. In the future, these WGs are expected to be one of the largest producers of renewable energy worldwide. In this paper, we propose a distributed control methodology for solving the problem of coordinating and controlling a group of SoA WGs to attain fast wind farm (WF) power output regulation with each storage device providing the same amount of power, i.e with equal sharing among the storage devices. Our proposed methodology introduces a consensus protocol for coordinating the grid-side converters (GSCs), whose dynamical equations constitute...

  19. Noncommutative numerical motives, Tannakian structures, and motivic Galois groups

    Marcolli, Matilde; Trigo Neri Tabuada, Goncalo Jorge
    In this article we further the study of noncommutative numerical motives, initiated in [30, 31]. By exploring the change-of-coefficients mechanism, we start by improving some of the main results of [30]. Then, making use of the notion of Schur-finiteness, we prove that the category NNum(k)F of noncommutative numerical motives is (neutral) super-Tannakian. As in the commutative world, NNum(k)F is not Tannakian. In order to solve this problem we promote periodic cyclic homology to a well-defined symmetric monoidal functor [over-bar HP∗]on the category of noncommutative Chow motives. This allows us to introduce the correct noncommutative analogues CNC and DNC of Grothendieck's...

  20. An algebraic interpretation of the multivariate q-Krawtchouk polynomials

    Post, Sarah; Vinet, Luc; Genest, Vincent
    The multivariate quantum q-Krawtchouk polynomials are shown to arise as matrix elements of “q-rotations” acting on the state vectors of many q-oscillators. The focus is put on the two-variable case. The algebraic interpretation is used to derive the main properties of the polynomials: orthogonality, duality, structure relations, difference equations, and recurrence relations. The extension to an arbitrary number of variables is presented.

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