Force-Extension Relations for Polymers with Sliding Links

Topological entanglements in polymers are mimicked by sliding rings (slip-links) which enforce pair contacts between monomers. We study the force-extension curve for linear polymers in which slip-links create additional loops of variable size. For a single loop in a phantom chain, we obtain exact expressions for the average end-to-end separation: The linear response to a small force is related to the properties of the unstressed chain, while for a large force the polymer backbone can be treated as a sequence of Pincus--de Gennes blobs, the constraint effecting only a single blob. Generalizing this picture, scaling arguments are used to include self-avoiding effects.Comment: 4 pages, 5 figures; accepted to Phys. Rev. E (Brief Report

Datos geocronológicos preliminares de la colisión entre el Gran Arco-Isla Caribeño y la Plataforma Continental de Bahamas en el Segmento de La Española

In parallel to structural and metamorphlc studies, geochronologlcal Investigations have been initiated in eclogites and blueschists from the Samaná Peninsula complex, northern Hispaniola, using Sm-Nd, UPb, Rb-Sr and 40Ar/39Ar methods, to constraint subduction, collision and exhumation processes in the Caribbean-North America plate boundary zone. A garnet-omphacite- whole rock Sm-Nd isochron from eclogite yields an age of 86±47 Ma (l43Nd/l44Ndinitial= 0,512894±0,000057; MSWD=0,00036). Although imprecise, we interpret this age as dating the peak of eclogitic metamorphism and allows us to calculate an initial eNd value of +7,2, which suggests a relatively depleted source for the protolith and that the rock formed in an intra-oceanic island-arc setting without significant influence from continental crust. Whole rock trace elements and REE data and preliminary Pb and Rb-Sr analyses also support this interpretation. The 40Ar/39Ar analyses of phengite mineral separates from eclogite and blueschist were made to record the cooling age during retrograde metamorphism (Tc»325-3009C). Phengites of eclogite and blueschist yields average plateau ages of 36,30±0,l 3 Ma and 33,85±0,12 Ma, respectively. These Late Eocene to Early Oligocène ages are related to the regional exhumation of the high-P basement complex and are attributed to the initial oblique collision of the Bahama Platform beneath the Hispaniola arc

Galaxy-scale Bars in Late-type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes

Galaxy-scale bars are expected to provide an effective means for driving material toward the central region in spiral galaxies, and possibly feeding supermassive black holes (BHs). Here we present a statistically complete study of the effect of bars on average BH accretion. From a well-selected sample of 50,794 spiral galaxies (with {M}_{* }\sim 0.2\mbox{--}30\times {10}^{10}\,{M}_{\odot }) extracted from the Sloan Digital Sky Survey Galaxy Zoo 2 project, we separate those sources considered to contain galaxy-scale bars from those that do not. Using archival data taken by the Chandra X-ray Observatory, we identify X-ray luminous (${L}_{{\rm{X}}}\gtrsim {10}^{41}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$) active galactic nuclei and perform an X-ray stacking analysis on the remaining X-ray undetected sources. Through X-ray stacking, we derive a time-averaged look at accretion for galaxies at fixed stellar mass and star-formation rate, finding that the average nuclear accretion rates of galaxies with bar structures are fully consistent with those lacking bars (${\dot{M}}_{\mathrm{acc}}\approx 3\times {10}^{-5}$ ${M}_{\odot }$ yr−1). Hence, we robustly conclude that large-scale bars have little or no effect on the average growth of BHs in nearby ($z\lt 0.15$) galaxies over gigayear timescales

On the distribution of bids for construction contract auctions

The statistical distribution representing bid values constitutes an essential part of many auction models and has involved a wide range of assumptions, including the Uniform, Normal, Lognormal and Weibull densities. From a modelling point of view, its goodness is defined by how well it enables the probability of a particular bid value to be estimated – a past bid for ex-post analysis and a future bid for ex-ante (forecasting) analysis. However, there is no agreement to date of what is the most appropriate form and empirical work is sparse. Twelve extant construction datasets from four continents over different time periods are analysed in this paper for their fit to a variety of candidate statistical distributions assuming homogeneity of bidders (ID not known). The results show there is no one single fit-all distribution, but that the 3p Log-Normal, Fréchet/2p Log-Normal, Normal, Gamma and Gumbel generally rank the best ex-post, and the 2p Log-Normal, Normal, Gamma and Gumbel the best ex-ante – with ex-ante having around three to four times worse fit than ex-post. Final comments focus on the results relating to the third and fourth standardised moments of the bids and a post-hoc rationalisation of the empirical outcome of the analysis

Alpha scattering and capture reactions in the A = 7 system at low energies

Differential cross sections for $^3$He-$\alpha$ scattering were measured in the energy range up to 3 MeV. These data together with other available experimental results for $^3$He $+ \alpha$ and $^3$H $+ \alpha$ scattering were analyzed in the framework of the optical model using double-folded potentials. The optical potentials obtained were used to calculate the astrophysical S-factors of the capture reactions $^3$He$(\alpha,\gamma)^7$Be and $^3$H$(\alpha,\gamma)^7$Li, and the branching ratios for the transitions into the two final $^7$Be and $^7$Li bound states, respectively. For $^3$He$(\alpha,\gamma)^7$Be excellent agreement between calculated and experimental data is obtained. For $^3$H$(\alpha,\gamma)^7$Li a $S(0)$ value has been found which is a factor of about 1.5 larger than the adopted value. For both capture reactions a similar branching ratio of $R = \sigma(\gamma_1)/\sigma(\gamma_0) \approx 0.43$ has been obtained.Comment: submitted to Phys.Rev.C, 34 pages, figures available from one of the authors, LaTeX with RevTeX, IK-TUW-Preprint 930540

Isolated ileal blind loop inflammation after intestinal resection with ileocolonic anastomosis in Crohn's disease: an often neglected endoscopic finding with an unfavorable outcome

OBJECTIVE: Postoperative endoscopic recurrence in patients with Crohn's disease (CD) is commonly classified using the Rutgeerts score. Ulcerations in the ileal blind loop are not taken into account in the Rutgeerts score, and the clinical relevance of these lesions is unknown. This study aimed to assess the outcome of isolated ileal blind loop inflammation (IBLI) in postoperative CD patients. METHODS: Adult CD patients who underwent intestinal surgery with ileocolonic anastomosis between 1997 and 2017 were included and postoperative endoscopy reports were retrospectively reviewed. IBLI was defined as isolated inflammation of the ileal blind loop with or without ulcera confined to the anastomosis. Outcome was assessed using endoscopic recurrence (Rutgeerts >i2) and surgical recurrence (re-resection). RESULTS: A total of 341 CD patients were included. In 125 out of 341 (37%) patients, the ileal blind loop was described in the endoscopy reports. IBLI was reported in 43 of 341 (13%) patients. Start or step-up drug therapy was initiated in 10 of 32 (31%) IBLI patients with abdominal symptoms within a median of 0.9 months [interquartile range (IQR) 0.7-1.4] after ileocolonoscopy. Endoscopic recurrence occurred in 4 out of 38 (11%) IBLI patients without re-resection, within a median of 12.4 months (IQR 6.8-13.3). Intestinal re-res

Unit bar architecture in a highly‐variable fluvial discharge regime: Examples from the Burdekin River, Australia

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development

Resummation of the Divergent Perturbation Series for a Hydrogen Atom in an Electric Field

We consider the resummation of the perturbation series describing the energy displacement of a hydrogenic bound state in an electric field (known as the Stark effect or the LoSurdo-Stark effect), which constitutes a divergent formal power series in the electric field strength. The perturbation series exhibits a rich singularity structure in the Borel plane. Resummation methods are presented which appear to lead to consistent results even in problematic cases where isolated singularities or branch cuts are present on the positive and negative real axis in the Borel plane. Two resummation prescriptions are compared: (i) a variant of the Borel-Pade resummation method, with an additional improvement due to utilization of the leading renormalon poles (for a comprehensive discussion of renormalons see [M. Beneke, Phys. Rep. vol. 317, p. 1 (1999)]), and (ii) a contour-improved combination of the Borel method with an analytic continuation by conformal mapping, and Pade approximations in the conformal variable. The singularity structure in the case of the LoSurdo-Stark effect in the complex Borel plane is shown to be similar to (divergent) perturbative expansions in quantum chromodynamics.Comment: 14 pages, RevTeX, 3 tables, 1 figure; numerical accuracy of results enhanced; one section and one appendix added and some minor changes and additions; to appear in phys. rev.

Governing Boards and Profound Organizational Change in Hospitals

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69047/2/10.1177_107755878904600204.pd