A solution for two-dimensional Fredholm integral equations of the second kind with periodic, semiperiodic, or nonperiodic kernels

A numerical scheme for solving two dimensional Fredholm integral equations of the second kind is developed. The proof of the convergence of the numerical scheme is shown for three cases: the case of periodic kernels, the case of semiperiodic kernels, and the case of nonperiodic kernels. Applications to the incompressible, stationary Navier-Stokes problem are of primary interest

A numerical solution for two-dimensional Fredholm integral equations of the second kind with kernels of the logarithmic potential form

Two dimensional Fredholm integral equations with logarithmic potential kernels are numerically solved. The explicit consequence of these solutions to their true solutions is demonstrated. The results are based on a previous work in which numerical solutions were obtained for Fredholm integral equations of the second kind with continuous kernels

Forecasting Value-at-Risk with Time-Varying Variance, Skewness and Kurtosis in an Exponential Weighted Moving Average Framework

This paper provides an insight to the time-varying dynamics of the shape of the distribution of financial return series by proposing an exponential weighted moving average model that jointly estimates volatility, skewness and kurtosis over time using a modified form of the Gram-Charlier density in which skewness and kurtosis appear directly in the functional form of this density. In this setting VaR can be described as a function of the time-varying higher moments by applying the Cornish-Fisher expansion series of the first four moments. An evaluation of the predictive performance of the proposed model in the estimation of 1-day and 10-day VaR forecasts is performed in comparison with the historical simulation, filtered historical simulation and GARCH model. The adequacy of the VaR forecasts is evaluated under the unconditional, independence and conditional likelihood ratio tests as well as Basel II regulatory tests. The results presented have significant implications for risk management, trading and hedging activities as well as in the pricing of equity derivatives

Measuring market liquidity: An introductory survey

Asset liquidity in modern financial markets is a key but elusive concept. A market is often said to be liquid when the prevailing structure of transactions provides a prompt and secure link between the demand and supply of assets, thus delivering low costs of transaction. Providing a rigorous and empirically relevant definition of market liquidity has, however, provided to be a difficult task. This paper provides a critical review of the frameworks currently available for modelling and estimating the market liquidity of assets. We consider definitions that stress the role of the bid-ask spread and the estimation of its components that arise from alternative sources of market friction. In this case, intra-daily measures of liquidity appear relevant for capturing the core features of a market, and for their ability to describe the arrival of new information to market participants.

Validation of water flux and body composition in Glaucous gulls (Larus hyperboreus)

Water influx rates (WIR) measured with tritiated water dilution were compared with direct measures of water and energy intake in glaucous gulls (Larus hyperboreus). Total body water (TBW) measured isotopically was also compared with TBW determined by body composition analysis (BCA) of the same birds. Seventeen wild gulls were captured and studied in outdoor enclosures at Ny-Ålesund, Svalbard, in July 2002. Gulls were hand-fed known quantities of Arctic cod (Boreogadus saida) or given water on the basis of one of four experimental treatments: (A) fasting, (B) fish only, (C) water only, or (D) fish and water. Water and energy content of Arctic cod was also determined. WIR of gulls (after subtracting metabolic water production) in treatments A, B, C, and D were 0, 101 ± 5, 62 ± 19, and 122 ± 21 SD g d-1, respectively. Measured water intake in each group was 0, 111 ± 2, 64 ± 3, and 134 ± 15 SD g d-1, respectively. On average, WIR underestimated measured water intake in each group. Errors were lowest but most variable for gulls fed water only (-2.2% ± 32.8%) compared with gulls fed fish only (-9.0% ± 5.4%) or fish and water (-9.0% ± 7.0%). Compared with measured water intake, errors in WIR were relatively low overall (-6.9% ± 17.4%) and comparable to previous validation studies. The difference in TBW determined by BCA versus isotopic dilution ranged between -1.02% and +8.59% of mass. On average, TBW measured isotopically (632 ± 24 g kg-1) overestimated true body water by a factor of 1.033

Novel Access to Known and Unknown Thiourea Catalyst via a Multicomponent-Reaction Approach

Thioureas are frequently used in organocatalysis and typically rely on 3,5‐bis(trifluoromethyl) phenyl moieties motifs to enhance their catalytic activity. In this work, these common motifs were replaced with tailorable functional groups, such as ester or sulfone aryls, applying elemental sulfur in a multicomponent reaction (MCR) strategy for the first time for thiourea catalyst synthesis. First, several thioureas bearing aryl, benzylic or aliphatic moieties were synthesized and tested for their hydrogen bonding strength by evaluating thiourea phosphine oxide complexes via $^{31}$P NMR and their catalytic activity in an Ugi four‐component reaction (U‐4CR). Finally, ester and sulfone aryl thioureas were tested in the aminolysis of propylene carbonate, leading to conversions similar to those previously reported in the literature using the 3,5‐bis(trifluoromethyl)phenyl moiety, proving that these groups are suitable alternatives for the trifluoromethyl group

Multiple stressors in a top predator seabird: potential ecological consequences of environmental contaminants, population health and breeding conditions

Environmental contaminants may have impacts on reproduction and survival in wildlife populations suffering from multiple stressors. This study examined whether adverse effects of persistent organic pollutants (POPs) increased with poor population health and breeding conditions in three colonies (60–74°N) of great skua (Stercorarius skua) in the north-eastern Atlantic (Shetland, Iceland and Bjørnøya [Bear Island]). POPs (organochlorines [OCs] and polybrominated diphenyl ethers [BDEs]) were measured in plasma of incubating birds (n = 222), concentrations differing nearly tenfold among colonies: Bjørnøya (2009) > Bjørnøya (2010) > Iceland (2009) > Shetland (2009). Reproductive success (hatching success and chick survival) showed that breeding conditions were favourable in Shetland and at Bjørnøya (2010), but were very poor in Iceland and at Bjørnøya (2009). Biomarkers indicated that health was poor in the Shetland population compared to the other populations. Females whose chicks hatched late had high POP concentrations in all colonies except at Bjørnøya (2010), and females losing their eggs at Bjørnøya (2009) tended to have higher concentrations than those hatching. Moreover, there was a negative relationship between female POP concentrations and chick body condition at hatching in Iceland and at Bjørnøya (2010). Supplementary feeding experiments were conducted, and in Iceland where feeding conditions were poor, significant negative relationships were found between female POP concentrations and daily growth-rate in first-hatched chicks of control nests, but not in food supplemented nests. This suggests that negative impacts of POPs were mitigated by improved feeding conditions. For second-chicks, there was a strong negative relationship between the female POP concentrations and growth-rate, but no effects of supplementary feeding. Lowered adult return-rate between breeding seasons with increasing POP loads were found both at Bjørnøya (2009) and in Shetland, especially related to BDEs. This indicates stronger fitness consequences of POPs following seasons with very poor breeding conditions and/or high reproductive effort. This study suggests that the impacts of POPs may differ depending on population health and breeding conditions, and that even low concentrations of POPs could have ecological consequences during adverse circumstances. This is important with regard to risk assessment of biomagnifying contaminants in marine ecosystems

A general strategy for discovery of inhibitors and activators of RING and U-box E3 ligases with ubiquitin variants

RING and U-box E3 ubiquitin ligases regulate diverse eukaryotic processes and have been implicated in numerous diseases, but targeting these enzymes remains a major challenge. We report the development of three ubiquitin variants (UbVs), each binding selectively to the RING or U-box domain of a distinct E3 ligase: monomeric UBE4B, phosphorylated active CBL, or dimeric XIAP. Structural and biochemical analyses revealed that UbVs specifically inhibited the activity of UBE4B or phosphorylated CBL by blocking the E2∼Ub binding site. Surprisingly, the UbV selective for dimeric XIAP formed a dimer to stimulate E3 activity by stabilizing the closed E2∼Ub conformation. We further verified the inhibitory and stimulatory functions of UbVs in cells. Our work provides a general strategy to inhibit or activate RING/U-box E3 ligases and provides a resource for the research community to modulate these enzymes

A more sustainable isothiocyanate synthesis by amine catalyzed sulfurization of isocyanides with elemental sulfur

Isothiocyanates (ITCs) are typically prepared using amines and highly toxic reagents such as thiophosgene, its derivatives, or CS$_{2}$. In this work, an investigation of a multicomponent reaction (MCR) using isocyanides, elemental sulfur and amines revealed that isocyanides can be converted to isothiocyanates using sulfur and catalytic amounts of amine bases, especially DBU (down to 2 mol%). This new catalytic reaction was optimized in terms of sustainability, especially considering benign solvents such as Cyrene™ or γ-butyrolactone (GBL) under moderate heating (40 °C). Purification by column chromatography was further optimized to generate less waste by maintaining high purity of the product. Thus, E-factors as low as 0.989 were achieved and the versatility of this straightforward procedure was shown by converting 20 different isocyanides under catalytic conditions, while obtaining moderate to high yields (34–95%)

Structure of UBE2K-Ub/E3/polyUb reveals mechanisms of K48-linked Ub chain extension

Ubiquitin (Ub) chain types govern distinct biological processes. K48-linked polyUb chains target substrates for proteasomal degradation, but the mechanism of Ub chain synthesis remains elusive due to the transient nature of Ub-handover. Here, we present the structure of a chemically trapped complex of E2 UBE2K covalently linked to donor Ub and acceptor K48-linked di-Ub, primed for K48-linked Ub chain synthesis by a RING E3. The structure reveals the basis for acceptor Ub recognition by UBE2K active site residues and the C-terminal Ub-associated (UBA) domain to impart K48-linked Ub specificity and catalysis. Furthermore, the structure unveils multiple Ub binding surfaces on the UBA domain that allow distinct binding modes for K48-linked and K63-linked Ub chains. This multivalent Ub binding feature serves to recruit UBE2K to ubiquitinated substrates to overcome weak acceptor Ub affinity and thereby promote chain elongation. These findings elucidate the mechanism of processive K48-linked polyUb chain formation by UBE2K