Look before you leap: a confidence-based method for selecting species criticality while avoiding negative populations in τ\tau-leaping

The stochastic simulation algorithm was introduced by Gillespie and in a different form by Kurtz. There have been many attempts at accelerating the algorithm without deviating from the behavior of the simulated system. The crux of the explicit $\tau$-leaping procedure is the use of Poisson random variables to approximate the number of occurrences of each type of reaction event during a carefully selected time period, $\tau$. This method is acceptable providing the leap condition, that no propensity function changes “significantly” during any time-step, is met. Using this method there is a possibility that species numbers can, artificially, become negative. Several recent papers have demonstrated methods that avoid this situation. One such method classifies, as critical, those reactions in danger of sending species populations negative. At most, one of these critical reactions is allowed to occur in the next time-step. We argue that the criticality of a reactant species and its dependent reaction channels should be related to the probability of the species number becoming negative. This way only reactions that, if fired, produce a high probability of driving a reactant population negative are labeled critical. The number of firings of more reaction channels can be approximated using Poisson random variables thus speeding up the simulation while maintaining the accuracy. In implementing this revised method of criticality selection we make use of the probability distribution from which the random variable describing the change in species number is drawn. We give several numerical examples to demonstrate the effectiveness of our new metho

Status Updates Over Unreliable Multiaccess Channels

Applications like environmental sensing, and health and activity sensing, are supported by networks of devices (nodes) that send periodic packet transmissions over the wireless channel to a sink node. We look at simple abstractions that capture the following commonalities of such networks (a) the nodes send periodically sensed information that is temporal and must be delivered in a timely manner, (b) they share a multiple access channel and (c) channels between the nodes and the sink are unreliable (packets may be received in error) and differ in quality. We consider scheduled access and slotted ALOHA-like random access. Under scheduled access, nodes take turns and get feedback on whether a transmitted packet was received successfully by the sink. During its turn, a node may transmit more than once to counter channel uncertainty. For slotted ALOHA-like access, each node attempts transmission in every slot with a certain probability. For these access mechanisms we derive the age of information (AoI), which is a timeliness metric, and arrive at conditions that optimize AoI at the sink. We also analyze the case of symmetric updating, in which updates from different nodes must have the same AoI. We show that ALOHA-like access, while simple, leads to AoI that is worse by a factor of about 2e, in comparison to scheduled access

The assessment of long-term orbital debris models

Existing long-term orbital debris models are assessed as a first step in the Air Force's effort to develop an Air Force long-term orbital debris model which can perform the following functions: (1) operate with the necessary accuracy at the relevant altitudes and orbital parameters; (2) benefit from new Air Force and non-Air Force debris measurements; and (3) accommodate current and future Air Force space scenarios. Model assessment results are shown for the NASA engineering model. The status of the NASA EVOLVE model assessment is discussed

Function and dysfunction of fatty acid mobilization: a review

Western populations have a growing obesity epidemic due in part to excessive nutrient intake from high-fat diets, which are increasingly common. Overindulgence of nutrients is associated with a greater incidence of metabolic dysfunction and a greater risk for obesity, diabetes, hypertension, and other metabolic disorders that lower quality of life. Research in humans and animal models has improved our understanding of how excess circulating free fatty acids negatively impact the ability of muscle and other tissues to regulate nutrient uptake and utilization. It is generally accepted by the scientific community that excess circulating fatty acids lead to insulin resistance, but there is little clarity regarding the underlying mechanisms. In the present review, we will outline the current understanding of the characteristics associated with fatty acid mobilization and fatty acid utilization within specific tissues. We will also discuss the potential mechanistic role of hyperlipidemia on metabolic dysfunction associated with type 2 diabetes

The M. W. Burks Site (41WD52): A Late Caddo Hamlet in Wood County, Texas

While attempting to locate and evaluate prehistoric Caddo archaeological sites in the Dry Creek watershed, Wood County, Texas, that had been originally recorded by A. T. Jackson and M. M. Reese in 1930, the M. W. Burks site (41WD52) was discovered by James E. Bruseth and Bob D. Skiles in June 1977. The site is in the Forest Hill community, about 5 km north of Quitman, Texas, in the East Texas Pineywoods and Gulf Coastal Plain. It is on a small rise in the uplands overlooking a small intermittent drainage that is an unnamed tributary of Little Dry Creek. The landowner, Mr. M. W. Burks, had resided in this part of Wood County since the 1920s, and recalled where A. T. Jackson and crew had spent time excavating the J. H. Reese (41WD2) site. He mentioned that while putting in a fence on his property in the early 1960s, adjacent to the property where the Reese site is located, he had found some pottery sherds in one of the post holes. Bruseth and Skiles placed a small shovel test next to this fence post hole, and a large articulated red-slipped Ripley Engraved carinated bowl was encountered at 65 em below the surface (bs) in tan sand E-horizon deposits. This find demonstrated that the Burks site contained both intact archaeological deposits as well as an apparently undisturbed Late Caddo Titus phase burial or cemetery. Bruseth, Skiles, and Perttula followed up this work with more intensive investigations in the spring and fall of 1978. This research was carried on as an adjunct to the ongoing (and final season of) archaeological work being conducted by Bruseth and Perttula at Lake Fork Reservoir on Lake Fork Creek, a few miles to the west of the Burks site. Our purpose in carrying out archaeological research at the Burks site was to examine in more detail the spatial character of a Late Caddo Titus phase settlement, and also obtain information on the material culture remains (especially the ceramics) made and used by the Caddo peoples that lived at the Burks site some 400-500 years ago

The Carlisle Site (41WD46), a Middle Caddoan Occupation on the Sabine River, Wood County, Texas

The Carlisle site (41WD46) is located on the Sabine River near its confluence with Lake Fork Creek in the Upper Sabine River Basin. As defined by Perttula, the Upper Sabine River Basin includes the area from the headwaters of the Sabine River to the mouths of Cherokee Bayou and Hatley Creek at the western edge of the Sabine Uplift. Lake Fork Creek is one of several large south-southeastward flowing streams within the Upper Sabine River Basin. The town of Mineola is approximately 13 kilometers (km) west of the Carlisle site. The site is situated at the tip of an upland projection overlooking the Sabine River floodplain, but extends into the floodplain to within ca 30 meters of the river bank. The Lake Fork Creek channel is approximately one km east of the site. While the site was an improved pasture for many years prior to 1975 and to the present, it had been previously cultivated. In fact, this cultivation may have contributed to its initial identification in the early 1930s, as well as its subsequent partial burial. The upland sandy soils derive from the Queen City Formation, and these are highly susceptible to erosion and colluvial downwasting. Colluvial deposition seems to have been a prominent factor in the burial of cultural materials along valley margins and lower footslopes elsewhere in the Upper Sabine Basin, and the site\u27s topographic position suggests that both alluvial and colluvial deposition is responsible for the burial of the floodplain cultural deposits at the Carlisle site

A simulation model of time-dependent plasma-spacecraft interactions

A plasma simulation code is presented that models the time-dependent plasma properties in the vicinity of a spherical, charged spacecraft. After showing agreement with analytic, steady-state theories and ATS-6 satellite data, the following three problems are treated: (1) transient pulses from photoemission at various emission temperatures and ambient plasma conditions, (2) spacecharge limited emission, and (3) simulated plasma oscillations in the long wavelength limit