University of California

About: University of California is a education organization based out in Oakland, California, United States. It is known for research contribution in the topics: Population & Layer (electronics). The organization has 55175 authors who have published 52933 publications receiving 1491169 citations. The organization is also known as: UC & University of California System.

The Spectrum in the Sachdev-Ye-Kitaev Model

Abstract: The SYK model consists of N ≫ 1 fermions in 0 + 1 dimensions with a random, all-to-all quartic interaction. Recently, Kitaev has found that the SYK model is maximally chaotic and has proposed it as a model of holography. We solve the SchwingerDyson equation and compute the spectrum of two-particle states in SYK, finding both a continuous and discrete tower. The four-point function is expressed as a sum over the spectrum. The sum over the discrete tower is evaluated.

Influence of high latitude ice cover on the marine Intertropical Convergence Zone

Abstract: We investigate the causes for a strong high latitude imposed ice (land or sea) influence on the marine Intertropical Convergence Zone (ITCZ) in the Community Climate Model version 3 coupled to a 50-m slab ocean. The marine ITCZ in all the ocean basins shift meridionally away from the hemisphere with an imposed added ice cover, altering the global Hadley circulation with an increased tropical subsidence in the hemisphere with imposed ice and uplift in the other. The effect appears to be independent of the longitudinal position of imposed ice. The anomalous ice induces a rapid cooling and drying of the air and surface over the entire high- and midlatitudes; subsequent progression of cold anomalies occurs in the Pacific and Atlantic northeasterly trade regions, where a wind-evaporation-sea surface temperature (SST) feedback initiates progression of a cold SST ‘front’ towards the ITCZ latitudes. Once the cooler SST reaches the ITCZ latitude, the ITCZ shifts southwards, aided by positive feedbacks associated with the displacement. The ITCZ displacement transports moisture away from the colder and drier hemisphere into the other hemisphere, resulting in a pronounced hemispheric asymmetric response in anomalous specific humidity; we speculate that the atmospheric humidity plays a central role in the hemispheric asymmetric nature of the climate response to high latitude ice cover anomalies. From an energy balance viewpoint, the increased outgoing radiative flux at the latitudes of the imposed ice is compensated by an increased radiative energy flux at the tropical latitudes occupied by the displaced ITCZ, and subsequently transported by the altered Hadley and eddy circulations to the imposed ice latitudes. The situation investigated here may be applicable to past climates like the Last Glacial Maximum where hemispheric asymmetric changes to ice cover occurred. Major caveats to the conclusions drawn include omission of interactive sea ice physics and ocean dynamical feedback and sensitivity to atmospheric physics parameterizations across different models.

Pretreatment assessment of resectable and borderline resectable pancreatic cancer: expert consensus statement.

Abstract: Preoperative Staging and Defining Resectability From a surgical perspective, the first objective in the management of suspected or confirmed pancreatic cancer is to determine the potential for resection. Routine exploratory laparotomy for the purpose of operatively determining resectability has been diminished by modern 3-D radiographic imaging, along with effective and sustainable nonoperative methods of palliation. Careful correlation between preoperative CT findings and surgical results has better-defined CT criteria for resectability. The critical aspects that need be evaluated in a thorough radiographic assessment are the presence or absence of peritoneal or hepatic metastases; the potential involvement of the SMV and portal vein and the relationship of these vessels and their tributaries to the tumor; the relationship of the tumor to the SMA, celiac axis, hepatic artery, and gastroduodenal artery; and the presence of any aberrant vascular anatomy. Unequivocal radiographic findings contraindicating resection include distant metastases, major venous thrombosis of the portal vein or SMV extending for several centimeters, and circumferential encasement of the SMA, celiac axis or proximal hepatic artery. Recent revisions of the National Comprehensive Cancer Network (NCCN) guidelines were an attempt to distinguish locally advanced unresectable tumors from potentially resectable tumors.22 Ambiguity exists in these guidelines because of the lack of clarity in defining clearly resectable situations from “borderline resectable” tumors and because of the subjective criteria used to define “borderline” tumors relative to locally advanced, unresectable lesions. The NCCN guidelines do offer a definition of what should be considered a radiographically resectable tumor. Patients without distant metastases and no evidence of tumor extension to the SMV and portal vein and clear fat planes around the celiac axis, the hepatic artery, and SMA should be categorized as having localized and resectable cancers. More refined and objective criteria have been proposed by the M. D. Anderson Cancer Center Pancreas Cancer Group in an attempt to better define the term “borderline resectable” and to guide treatment decisions regarding the use of neoadjuvant therapy and the high likelihood of vein resection and reconstruction as a means to improve the rate of a complete and margin-negative resection.23 Radiographic findings of tumor abutment on the portal vein or SMV with or without venous deformity, and limited encasement of the mesenteric vein and portal vein (i.e., short segment occlusion with suitable vessel for anastomosis above and below) represent the extent of venous involvement that would categorize a tumor as borderline resectable. Radiographic findings suggesting borderline arterial involvement as defined by M. D. Anderson Cancer Center include encasement of a short segment of the hepatic artery, without evidence of tumor extension to the celiac axis and/or tumor abutment of the SMA involving < 180° of the artery circumference. In patients without clinically important major comorbidities, and in the absence of radiographic findings to suggest metastatic disease or locally advanced unresectable disease as outlined above, surgical resection should be considered feasible and likely to be achievable. Whether these resections would result in a higher-than-expected rate of margin-positive resections, and whether such resections would affect survival would best be determined by careful examination of outcomes relative to extent of vascular involvement using objective criteria to determine categorization of extent of disease. Consensus Statement 1. Tumors considered localized and resectable should demonstrate the following: a. No distant metastases. b. No radiographic evidence of SMV and portal vein abutment, distortion, tumor thrombus, or venous encasement. c. Clear fat planes around the celiac axis, hepatic artery, and SMA. 2. Tumors considered borderline resectable include the following: a. No distant metastases. b. Venous involvement of the SMV/portal vein demonstrating tumor abutment with or without impingement and narrowing of the lumen, encasement of the SMV/portal vein but without encasement of the nearby arteries, or short segment venous occlusion resulting from either tumor thrombus or encasement but with suitable vessel proximal and distal to the area of vessel involvement, allowing for safe resection and reconstruction. c. Gastroduodenal artery encasement up to the hepatic artery with either short segment encasement or direct abutment of the hepatic artery, without extension to the celiac axis. d. Tumor abutment of the SMA not to exceed >180° of the circumference of the vessel wall.

Upper Palaeolithic Siberian genome reveals dual ancestry of Native Americans

Abstract: The origins of the First Americans remain contentious. Although Native Americans seem to be genetically most closely related to east Asians, there is no consensus with regard to which specific Old World populations they are closest to. Here we sequence the draft genome of an approximately 24,000-year-old individual (MA-1), from Mal'ta in south-central Siberia, to an average depth of 1×. To our knowledge this is the oldest anatomically modern human genome reported to date. The MA-1 mitochondrial genome belongs to haplogroup U, which has also been found at high frequency among Upper Palaeolithic and Mesolithic European hunter-gatherers, and the Y chromosome of MA-1 is basal to modern-day western Eurasians and near the root of most Native American lineages. Similarly, we find autosomal evidence that MA-1 is basal to modern-day western Eurasians and genetically closely related to modern-day Native Americans, with no close affinity to east Asians. This suggests that populations related to contemporary western Eurasians had a more north-easterly distribution 24,000 years ago than commonly thought. Furthermore, we estimate that 14 to 38% of Native American ancestry may originate through gene flow from this ancient population. This is likely to have occurred after the divergence of Native American ancestors from east Asian ancestors, but before the diversification of Native American populations in the New World. Gene flow from the MA-1 lineage into Native American ancestors could explain why several crania from the First Americans have been reported as bearing morphological characteristics that do not resemble those of east Asians. Sequencing of another south-central Siberian, Afontova Gora-2 dating to approximately 17,000 years ago, revealed similar autosomal genetic signatures as MA-1, suggesting that the region was continuously occupied by humans throughout the Last Glacial Maximum. Our findings reveal that western Eurasian genetic signatures in modern-day Native Americans derive not only from post-Columbian admixture, as commonly thought, but also from a mixed ancestry of the First Americans.

Coordination and Lock-In: Competition with Switching Costs and Network Effects

Abstract: Switching costs and network effects bind customers to vendors if products are incompatible, locking customers or even markets in to early choices. Lock-in hinders customers from changing suppliers in response to (predictable or unpredictable) changes in efficiency, and gives vendors lucrative ex post market power – over the same buyer in the case of switching costs (or brand loyalty), or over others with network effects. Firms compete ex ante for this ex post power, using penetration pricing, introductory offers, and price wars. Such “competition for the market” or “life-cycle competition” can adequately replace ordinary compatible competition, and can even be fiercer than compatible competition by weakening differentiation. More often, however, incompatible competition not only involves direct efficiency losses but also softens competition and magnifies incumbency advantages. With network effects, established firms have little incentive to offer better deals when buyers' and complementors' expectations hinge on non-efficiency factors (especially history such as past market shares), and although competition between incompatible networks is initially unstable and sensitive to competitive offers and random events, it later “tips” to monopoly, after which entry is hard, often even too hard given incompatibility. And while switching costs can encourage small-scale entry, they discourage sellers from raiding one another's existing customers, and so also discourage more aggressive entry. Because of these competitive effects, even inefficient incompatible competition is often more profitable than compatible competition, especially for dominant firms with installed-base or expectational advantages. Thus firms probably seek incompatibility too often. We therefore favor thoughtfully pro-compatibility public policy.