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Caltech Authors (128.467 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 5.893

  1. A First-Principles Approach to Understanding the Internet’s Router-level Topology

    Li, Lun; Alderson, David; Willinger, Walter; Doyle, John
    A detailed understanding of the many facets of the Internet’s topological structure is critical for evaluating the performance of networking protocols, for assessing the effectiveness of proposed techniques to protect the network from nefarious intrusions and attacks, or for developing improved designs for resource provisioning. Previous studies of topology have focused on interpreting measurements or on phenomenological descriptions and evaluation of graph-theoretic properties of topology generators. We propose a complementary approach of combining a more subtle use of statistics and graph theory with a first-principles theory of router-level topology that reflects practical constraints and tradeoffs. While there is an inevitable tradeoff between model complexity and...

  2. Using Spinning Dust Emission To Constrain The Abundance Of Very Small Dust Grains In Dense Cores

    Tibbs, C. T.; Paladini, R.; Cleary, K.; Muchovej, S.; Scaife, A.; Stevenson, M.; Laureijs, R.; Ysard, N.; Grainge, K.; Perrott, Y.; Rumsey, C.; Villadsen, J.
    We present the first analysis of using spinning dust emission as a method to characterise the properties of very small interstellar dust grains in dense cores.

  3. Explosive Outflows from Forming Massive Stars

    Bally, J.; Ginsburg, A.; Kasliwal, M. M.
    AO imaging of the near IR [Fe ii] and H_2 lines and ALMA CO J = 2 − 1 data confirms the explosive nature of the BN/KL outflow in Orion. N-body interactions in compact groups may be responsible for the production of powerful, explosive protostellar outflows and luminous infrared flares. The Orion event may have been triggered by a protostellar merger. First results of a search for Orion-like events in 200 nearby galaxies with the SPitzer InfraRed Intensive Transients Survey (SPIRITS) are briefly discussed.

  4. Continuum radiative transfer Modeling of Sagittarius B2

    Schmiedeke, A.; Schilke, P.; Möller, Th.; Sánchez-Monge, Á.; Bergin, E.; Comito, C.; Csengeri, T.; Lis, D. C.; Molinari, S.; Qin, S. L.; Rolffs, R.
    We present results from radiative transfer modeling of the continuum emission towards Sagittarius B2 (hereafter Sgr B2). We have developed a radiative transfer framework – Pandora – that employs RADMC-3D (Dullemond 2012) for a self-consistent determination of the dust temperature. With this pipeline, we have set-up a single model that consistently reproduces the thermal dust and free-free continuum emission of Sgr B2 spanning four orders of magnitude in spatial scales (0.02–45 pc) and two orders of magnitude in frequency (20–4000 GHz).

  5. PATHA: Performance Analysis Tool for HPC Applications

    Yoo, Wucherl; Koo, Michelle; Cao, Yi; Sim, Alex; Nugent, Peter; Wu, Kesheng
    Large science projects rely on complex workflows to analyze terabytes or petabytes of data. These jobs are often running over thousands of CPU cores and simultaneously performing data accesses, data movements, and computation. It is difficult to identify bottlenecks or to debug the performance issues in these large workflows. To address these challenges, we have developed Performance Analysis Tool for HPC Applications (PATHA) using the state-of-art open source big data processing tools. Our framework can ingest system logs to extract key performance measures, and apply the most sophisticated statistical tools and data mining methods on the performance data. It utilizes...

  6. A comparison of automorphic and Artin L-series of GL(2)-type agreeing at degree one primes

    Martin, Kimball; Ramakrishnan, Dinakar
    Let F/k be a cyclic extension of number fields of prime degree. Let ρ be an irreducible 2-dimensional representation of Artin type of the absolute Galois group of F, and π a cuspidal automorphic representation of GL_2(A_F), such that the L-functions L(s,ρ_v) and L(s,π_v) agree at all (but finitely many of) the places v of degree one over k. We prove in this case that we have the global identity L(s,ρ)=L(s,π), with ρ_v↔π_v being given by the local Langlands correspondence at all v. In particular, π is tempered and L(s,ρ) is entire.

  7. Multiparameter Photoacoustic Tomography

    Hu, Song; Favazza, Christopher; Wang, Lihong V.
    Physiology is the study of mechanical, physical, and chemical functions or organelles, cells, tissues, and organs in living systems. Disorders in physiological functions can lead to disease and death. Thus, studying physiology and pathophysiology in a quantitative manner is important for both fundamental and translational biomedicine.

  8. Observations of Loss-Cone Pitch Angle Distributions of Solar Energetic Particles

    Leske, R. A.; Cummings, A. C.; Cohen, C. M. S.; Mewaldt, R. A.; Labrador, A. W.; Stone, E. C.; Wiedenbeck, M. E.; Christian, E. R.; von Rosenvinge, T. T.
    Pitch angle distributions of solar energetic particles (SEPs) in interplanetary space sometimes exhibit a loss cone in which an incident beam of particles is partially mirrored; particles with large pitch angles are reflected while those with smaller pitch angles are not. Mirroring requires a magnetic field enhancement, but if the field strength is not large enough to turn around particles with the smallest pitch angles or if these particles scatter before reaching their more distant mirror points, a loss cone forms. Such distributions therefore provide information on the interplanetary environment far from the spacecraft. The Low Energy Telescopes onboard the...

  9. Fe Enhancements in SEP Onsets: Flare/CME Mixture or Transport Effect?

    Mason, G. M.; Li, G.; Cohen, C. M. S.; Desai, M. I.; Haggerty, D. K.; Leske, R. A.; Mewaldt, R. A.; Zank, G. P.
    During the onset phases of SEP events, the Fe/O ratio is often observed to be initially enhanced (˜1) over typical SEP values, followed by a decline to values close to typical averages over entire events (Fe/O ˜0.1). Two mechanisms have been suggested to explain this behavior, namely (1) a two-step process with an initial injection of “flare" particles with high Fe/O followed by shock-accelerated particles with lower Fe/O, and (2) a transport effect wherein the lower charge-to-mass ratio of Fe vs. O results in faster transport of Fe to the observer, leading to enhanced Fe/O in the early stages of...

  10. Smooth Imitation Learning for Online Sequence Prediction

    Le, Hoang M.; Kang, Andrew; Yue, Yisong; Carr, Peter
    We study the problem of smooth imitation learning for online sequence prediction, where the goal is to train a policy that can smoothly imitate demonstrated behavior in a dynamic and continuous environment in response to online, sequential context input. Since the mapping from context to behavior is often complex, we take a learning reduction approach to reduce smooth imitation learning to a regression problem using complex function classes that are regularized to ensure smoothness. We present a learning meta-algorithm that achieves fast and stable convergence to a good policy. Our approach enjoys several attractive properties, including being fully deterministic, employing...

  11. Learning Online Smooth Predictors for Realtime Camera Planning using Recurrent Decision Trees

    Chen, Jianhui; Le, Hoang M.; Carr, Peter; Yue, Yisong; Little, James J.
    We study the problem of online prediction for realtime camera planning, where the goal is to predict smooth trajectories that correctly track and frame objects of interest (e.g., players in a basketball game). The conventional approach for training predictors does not directly consider temporal consistency, and often produces undesirable jitter. Although post-hoc smoothing (e.g., via a Kalman filter) can mitigate this issue to some degree, it is not ideal due to overly stringent modeling assumptions (e.g., Gaussian noise). We propose a recurrent decision tree framework that can directly incorporate temporal consistency into a data-driven predictor, as well as a learning...

  12. Stability of Causal Inference

    Schulman, Leonard J.; Srivastava, Piyush
    We consider the sensitivity of causal identification to small perturbations in the input. A long line of work culminating in papers by Shpitser and Pearl (2006) and Huang and Valtorta (2008) led to a complete procedure for the causal identification problem. In our main result in this paper, we show that the identification function computed by these procedures is in some cases extremely unstable numerically. Specifically, the “condition number” of causal identification can be of the order of Ω(exp(n ^(0.49))) on an identifiable semiMarkovian model with n visible nodes. That is, in order to give an output accurate to d bits, the empirical probabilities of the observable events need to be obtained to accuracy d...

  13. Fe Enhancements in SEP Onsets: Flare/CME Mixture or Transport Effect?

    Mason, G. M.; Li, G.; Cohen, C. M. S.; Desai, M. I.; Haggerty, D. K.; Leske, R. A.; Mewaldt, R. A.; Zank, G. P.
    During the onset phases of SEP events, the Fe/O ratio is often observed to be initially enhanced (~1) over typical SEP values, followed by a decline to values close to typical averages over entire events (Fe/O ~0.1). Two mechanisms have been suggested to explain this behavior, namely (1) a two-step process with an initial injection of “flare” particles with high Fe/O followed by shock-accelerated particles with lower Fe/O, and (2) a transport effect wherein the lower charge-to-mass ratio of Fe vs. O results in faster transport of Fe to the observer, leading to enhanced Fe/O in the early stages of the event....

  14. The Time Variation of Energetic 3He in Interplanetary Space from 1997-2005

    Wiedenbeck, M. E.; Mason, G. M.; Cohen, C. M. S.; Cummings, A. C.; Dwyer, J. R.; Gold, R. E.; Krimigis, S. M.; Leske, R. A.; Mazur, J. E.; Mewaldt, R. A.; Stone, E. C.; von Rosenvinge, T. T.

  15. Campanella and the Disciplines from Obscurity to Concealment

    Haugen, Kristine Louise
    During 27 years in prison, Tommaso Campanella became a European celebrity by publishing natural philosophy, political philosophy, and more. What was it like to write in prison in the 17th century, and did those conditions leave any traces in his books? By comparison with the chaotic style of his earlier career, Campanella usually became clearer and more appealing, relying much less on books and more on original argument. On the other hand, his abiding interest in republican political thought demanded an alert and even sympathetic reader as Campanella ostensibly opposed republicanism with disputations and authorities that recalled his youth.

  16. Quantum Proofs

    Vidick, Thomas; Watrous, John
    Quantum information and computation provide a fascinating twist on the notion of proofs in computational complexity theory. For instance, one may consider a quantum computational analogue of the complexity class NP, known as QMA, in which a quantum state plays the role of a proof (also called a certificate or witness), and is checked by a polynomial-time quantum computation. For some problems, the fact that a quantum proof state could be a superposition over exponentially many classical states appears to offer computational advantages over classical proof strings. In the interactive proof system setting, one may consider a verifier and one...

  17. High-speed photoacoustic tomography

    Song, Liang; Guo, Zijian; Wang, Lihong V.
    Traditional diagnostic imaging technologies—including ultrasound, x-ray computed tomography, and magnetic resonance imaging—have dramatically improved patient care in the clinic. However, these technologies still have limited ability to detect small lesions. In most cases, the detectable lesions are ~1 cm in dimeter, corresponding to ~10^9 cells (including circulating and microscopic metastatic deposits) and representing ~2/3 of the natural history of a cancer [1]. Consequently, the window of opportunity for treatment before a tumor reaches its lethal state is narrow. Hence, it is imperative to develop novel technologies capable of detecting neoplasia early. Non ionizing biomedical optical imaging is highly desirable because of...

  18. A Dual-Modality Photoacoustic and Ultrasound Imaging System for Noninvasive Sentinel Lymph Node Detection: Preliminary Clinical Results

    Erpelding, Todd N.; Garcia-Uribe, Alejandro; Krumholz, Arie; Ke, Haixin; Maslov, Konstantin; Appleton, Catherine; Margenthaler, Julie; Wang, Lihong V.
    Sentinel lymph node biopsy (SLNB) has emerged as an accurate, less invasive alternative to axillary lymph node dissection, and it has rapidly become the standard of care for patients with clinically node-negative breast cancer. The sentinel lymph node (SLN) hypothesis states that the pathological status of the axilla can be accurately predicted by determining the status of the first (i.e., sentinel) lymph nodes that drain from the primary tumor. Physicians use radio-labeled sulfur colloid and/or methylene blue dye to identify the SLN, which is most likely to contain metastatic cancer cells. However, the surgical procedure causes morbidity and associated expenses. To overcome these limitations, we developed a...

  19. Probability 1 Computation with Chemical Reaction Networks

    Cummings, Rachel; Doty, David; Soloveichik, David
    The computational power of stochastic chemical reaction networks (CRNs) varies significantly with the output convention and whether or not error is permitted. Focusing on probability 1 computation, we demonstrate a striking difference between stable computation that converges to a state where the output cannot change, and the notion of limit-stable computation where the output eventually stops changing with probability 1. While stable computation is known to be restricted to semilinear predicates (essentially piecewise linear), we show that limitstable computation encompasses the set of predicates in Δ^0_2 in the arithmetical hierarchy (a superset of Turing-computable). In finite time, our construction achieves...

  20. Coarse-Grained Molecular Dynamics Simulations of the Bacterial Cell Wall

    Nguyen, Lam T.; Gumbart, James C.; Jensen, Grant J.
    Understanding mechanisms of bacterial sacculus growth is challenging due to the time and length scales involved. Enzymes three orders of magnitude smaller than the sacculus somehow coordinate and regulate their processes to double the length of the sacculus while preserving its shape and integrity, all over a period of tens of minutes to hours. Decades of effort using techniques ranging from biochemical analysis to microscopy have produced vast amounts of data on the structural and chemical properties of the cell wall, remodeling enzymes and regulatory proteins. The overall mechanism of cell wall synthesis, however, remains elusive. To approach this problem...

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