Browsing by Subject "systems"
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Item Open Access Active-Space N-Representability Constraints for Variational Two-Particle Reduced Density Matrix Calculations(2010) Shenvi, Neil; Izmaylov, Artur FThe ground-state energy of a system of fermions can be calculated by minimizing a linear functional of the two-particle reduced density matrix (2-RDM) if an accurate set of N-representability conditions is applied. In this Letter we introduce a class of linear N-representability conditions based on exact calculations on a reduced active space. Unlike wave-function-based approaches, the 2-RDM methodology allows us to combine information from calculations on different active spaces. By adding active-space constraints, we can iteratively improve our estimate for the ground-state energy. Applying our methodology to a 1D Hubbard model yields a significant improvement over traditional 2-positivity constraints with the same computational scaling.Item Open Access Bifurcation Analysis of Gene Regulatory Circuits Subject to Copy Number Variation(2010) Mileyko, Yuriy; Weitz, Joshua SGene regulatory networks are comprised of many small gene circuits. Understanding expression dynamics of gene circuits for broad ranges of parameter space may provide insight into the behavior of larger regulatory networks as well as facilitate the use of circuits as autonomous units performing specific regulatory tasks. In this paper, we consider three common gene circuits and investigate the dependence of gene expression dynamics on the circuit copy number. In particular, we perform a detailed bifurcation analysis of the circuits' corresponding nonlinear gene regulatory models restricted to protein-only dynamics. Employing a geometric approach to bifurcation theory, we are able to derive closed form expressions for conditions which guarantee existence of saddle-node bifurcations caused by variation in the circuit copy number or copy number concentration. This result shows that the drastic effect of copy number variation on equilibrium behavior of gene circuits is highly robust to variation in other parameters in the circuits. We discuss a possibility of extending the current results to higher dimensional models which incorporate more details of the gene regulatory process.Item Open Access Enhancing Mobile Devices through Code Offload(2012) Cuervo, EduardoAdvances in mobile hardware and operating systems have made mobile a first-class development platform. Activities such as web browsing, casual game play, media playback, and document reading are now as common on mobile devices as on full-sized desktop systems. However, developers are still constrained by the inherent resource limitations of mobile devices. Unlike desktop systems, mobile devices must sacrifice performance to accomodate smaller form factors and battery-backed operation. Opportunistic offloading of computation from a mobile device to remote server infrastructure (i.e., "code offload") offers a promising way to overcome these constraints and to expand the set of applications
(i.e., "apps") that can run on devices.
Deciding to offload requires a careful consideration of the costs and benefits of a range of possible program partitions. This cost-benefit analysis depends on external factors, such as network conditions and the resources availability, as well as internal app properties, such as component dependencies, data representations, and code complexity. Thus, benefiting from offload requires some assistance from developers, but requiring developers to adopt arcane or unnatural programming models will hinder adoption of regardless of the potential benefits.
In this dissertation we characterize two frameworks that reduce the amount of developer effort required to improve the performance of mobile apps through code offload. The first, MAUI, is designed for computationally intensive general-purpose apps such as speech
and facial recognition. The second, Kahawai, is designed for graphics-intensive apps like fast-action video games.
MAUI continuously monitors the device, network, and app, and uses its measurements to compute an energy-efficient program partition. MAUI reduces the burden on developers by taking advantage of core features of the managed code environments common to mobile
platforms: code portability, serialization, reflection, and type safety. These features allows MAUI to automatically instrument and potentially offload methods that the developer has tagged as suitable for offload. MAUI is particularly effective on applications composed by operations whose computational cost is large compared to the transfer cost of their input parameters and their output results.
Kahawai is designed for graphics-intensive apps such as console-style games and takes advantage of two features of today's mobile gaming platforms: capable mobile GPUs and reusable game engines. Even though today's mobile devices cannot duplicate the sophisticated graphical detail provided by gaming consoles and high-end desktop GPUs, devices have seen rapid improvements in their GPU processing capabilities. Kahawai leverages a device's GPU to provide collaborative rendering. Collaborative rendering relies on a mobile GPU to generate low-fidelity output, which when combined with server-side GPU output allows a mobile device to display a high-fidelity result. The benefits of collaborative rendering are substantial: mobile clients can experience high-quality graphical output using relatively little bandwidth. Fortunately, because most modern games are built on top of reusable game engines, developers only have to identify the sources of non-determinism
in the game logic to take advantage collaborative rendering.
Together, MAUI and Kahawai demonstrate that code offload can provide substantial benefits for mobile apps without overburdening app developers.
Item Open Access Spontaneous brain activity as a source of ideal 1/f noise(2009) Allegrini, Paolo; Menicucci, Danilo; Bedini, Remo; Fronzoni, Leone; Gemignani, Angelo; Grigolini, Paolo; West, Bruce J; Paradisi, PaoloWe study the electroencephalogram (EEG) of 30 closed-eye awake subjects with a technique of analysis recently proposed to detect punctual events signaling rapid transitions between different metastable states. After single-EEG-channel event detection, we study global properties of events simultaneously occurring among two or more electrodes termed coincidences. We convert the coincidences into a diffusion process with three distinct rules that can yield the same mu only in the case where the coincidences are driven by a renewal process. We establish that the time interval between two consecutive renewal events driving the coincidences has a waiting-time distribution with inverse power-law index mu approximate to 2 corresponding to ideal 1/f noise. We argue that this discovery, shared by all subjects of our study, supports the conviction that 1/f noise is an optimal communication channel for complex networks as in art or language and may therefore be the channel through which the brain influences complex processes and is influenced by them.Item Open Access The Ultraviolet Absorption Coefficient of Melanosomes Decreases with Increasing Pheomelanin Content(2010) Peles, Dana N; Simon, John DUveal melanosomes from the iridal stroma contain both eumelanin and pheomelanin, the ratio of which varies with iris color. Herein, we report the absorption coefficient at lambda = 244 nm for individual human iridal stroma melanosomes from dark brown and blue-green irides. The melanosomes are nearly identical in size, but differ in the relative concentration composition, ranging from a eumelanin/pheomelanin ratio of 14.8:1 (dark brown) to 1.3:1 (blue-green or hazel). The absorption coefficient of the melanosome decreases as its pheomelanin content increases. The origin of this decrease is attributed to a corresponding decrease in the number of UV-absorbing chromophores, reflecting the different molecular volumes of the monomeric building blocks of the two pigments. In agreement with reported data on synthetic pigments, the absorption coefficient of pheomelanin is found to be slightly larger than that for eumelanin at lambda = 244 nm (by a factor of 1.2). On the basis of the reported optical properties of synthetic models, this result suggests that the absorption of pheomelanin is less than eumelanin at wavelengths of biological relevance (similar to 315-400 nm).Item Open Access Wielding the Wand without Facing the Music: Allowing Utilization Review Physicians to Trump Doctors' Orders, but Protecting them from the Legal Risk Ordinarily Attached to the Medical Degree(2010) Record, Katherine LThis Note identifies a discrepancy in the law governing the decisionmaking that directs patient care. Seeking treatment that a third party will pay for, a patient needs not only a physician-prescribed course of treatment but also an insurer's verification that the cost is medically necessary or otherwise covered by the patient's plan. Both of these decisions directly impact the ultimate care delivered to the patient, but are governed by two very different liability regimes. A patient who suffers an adverse outcome may sue his physician in tort, while a patient who suffers from a lack of coverage may generally sue his insurer only under contract. In other words, when a patient suffers from inadequate care, his potential remedies vary considerably depending on whether the physician or the insurer is the defendant. This discrepancy in liability is the consequence of the federal law governing the administration of employer-sponsored health plans, and its extensive preemption of related state law. Many commentators have called for legal reform to address the distortion of managed care liability that results, arguing that managed care liability must be consistent or that wronged beneficiaries must have access to meaningful remedies. This Note argues that the federal law governing managed care organizations is problematic for a different reason and that the first step toward reform may be more elementary than previously suggested. Specifically, it suggests that the law governing insurers' coverage decisions is inconsistent with the law governing treatment recommendations. Patients suffer the same harm from error in both contexts-but because they can recover substantially more from treating physicians, doctors are named as defendants even when the insurers make errors. Further, this Note argues that simply aligning these two standards might offer a gateway to reform.