Analysis of a Splitting Estimator for Rare Event Probabilities in Jackson Networks

We consider a standard splitting algorithm for the rare-event simulation of overflow probabilities in any subset of stations in a Jackson network at level n, starting at a fixed initial position. It was shown in DeanDup09 that a subsolution to the Isaacs equation guarantees that a subexponential number of function evaluations (in n) suffice to estimate such overflow probabilities within a given relative accuracy. Our analysis here shows that in fact O(n^{2{\beta}+1}) function evaluations suffice to achieve a given relative precision, where {\beta} is the number of bottleneck stations in the network. This is the first rigorous analysis that allows to favorably compare splitting against directly computing the overflow probability of interest, which can be evaluated by solving a linear system of equations with O(n^{d}) variables.Comment: 23 page

International Borrowing, Capital Controls and the Exchange Rate: Lessons from Chile

This paper analyzes the Chilean experience with capital flows. We discuss the role played by capital controls, financial regulations and the exchange rate regime. The focus is on the period after 1990, the period when Chile returned to international capital markets. We also discuss the early 80s, where a currency collapse triggered a financial crisis in Chile, despite stricter capital controls on inflows than the 90s and tighter currency matching requirements on the banking sector. We conclude that financial regulation and the exchange rate regime are at the center of capital inflows experiences and financial vulnerabilities. Rigid exchange rates induce vulnerabilities, which may lead to sharp capital account reversals. We also discuss three important characteristics of the Chilean experience since the 90s. The first is the fact that most international borrowing is done directly by corporations and it is not intermediated by the banking system. The second is the implication of the free trade agreement of Chilean and the US regarding capital controls. Finally, we examine the Chilean experience following the Asian-Russia crisis, showing that Chile did not suffer a sudden-stop, but a current account reversal due to policy reactions and a sudden-start in capital outflows.

Iron Intercalation in Covalent-Organic Frameworks: A Promising Approach for Semiconductors

Covalent-organic frameworks (COFs) are intriguing platforms for designing functional molecular materials. Here, we present a computational study based on van der Waals dispersion-corrected hybrid density functional theory (DFT-D) to design boroxine-linked and triazine-linked COFs intercalated with Fe. Keeping the original $P-6m2$ symmetry of the pristine COF (COF-Fe-0), we have computationally designed seven new COFs by intercalating Fe atoms between two organic layers. The equilibrium structures and electronic properties of both the pristine and Fe-intercalated COF materials are investigated here. We predict that the electronic properties of COFs can be fine tuned by adding Fe atoms between two organic layers in their structures. Our calculations show that these new intercalated-COFs are promising semiconductors. The effect of Fe atoms on the electronic band structures and density of states (DOSs) has also been investigated using the aforementioned DFT-D method. The contribution of the $d$-subshell electron density of the Fe atoms plays an important role in improving the semiconductor properties of these new materials. These intercalated-COFs provide a new strategy to create semi-conducting materials within a rigid porous network in a highly controlled and predictable manner.Comment: 39 pages. arXiv admin note: text overlap with arXiv:1703.0261

UV spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analog

The Galactic blue supergiant SBW1 with its circumstellar ring nebula represents the best known analog of the progenitor of SN 1987A. High-resolution imaging has shown H-alpha and IR structures arising in an ionized flow that partly fills the ring's interior. To constrain the influence of the stellar wind on this structure, we obtained an ultraviolet (UV) spectrum of the central star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum shows none of the typical wind signatures, indicating a very low mass-loss rate. Radiative transfer models suggest an extremely low rate below 10$^{-10}$ Msun/yr, although we find that cooling timescales probably become comparable to or longer than the flow time below 10$^{-8}$ Msun/yr. We therefore adopt this latter value as a conservative upper limit. For the central star, the model yields $T_{\rm eff}$=21,000$\pm$1000 K, $L\simeq$5$\times$10$^4$ $L_{\odot}$, and roughly Solar composition except for enhanced N abundance. SBW1's very low mass-loss rate may hinder the wind's ability to shape the surrounding nebula. The very low mass-loss rate also impairs the wind's ability to shed angular momentum; the spin-down timescale for magnetic breaking is more than 500 times longer than the age of the ring. This, combined with the star's slow rotation rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at least 10 times lower than expected from mass-loss recipes, without any account of clumping. The physical explanation for why SBW1's wind is so weak presents an interesting mystery.Comment: 12 pages, 6 figs. submitted to MNRAS. comments welom

Financial Diversification, Sudden Stops and Sudden Starts

The recent literature on sudden stops is based on the fact that many emerging market economies experience recurrent and sharp capital account reversals. In this paper we argue, as some recent research has started to emphasize, that more information can be obtained by looking at gross rather than net flows. Economies may be curtailed from international financial markets, resulting in a sudden stop of inflows, but others may be experiencing portfolio shifts that cause sudden start of capital outflows. By looking at gross flows, and comparing emerging markets (EMEs) with developed economies (DEs) we indeed show that there is a variety of experiences that cannot be lumped together. In particular, sudden stop of inflows are as common in DEs as in EMEs, but a key difference is that in the former outflows and inflows are negatively correlated, which dampen the reversal of net flows. We present a model of financial diversification to interpret these results which is consistent with most evidence we report here. l II) could be helpful on this task.

Hyperacute Therapies for Childhood Stroke: A Case Report and Review of the Literature

Objective. The optimal management of pediatric patients with arterial ischemic stroke (AIS) is not known. Despite this, goal-oriented, time-sensitive therapies geared to rapid reestablishment of arterial blood flow are occasionally applied with beneficial effects. The inconsistent approach to AIS is in part due to a lack of knowledge and preparedness. Methods. Case report of a 12-year-old male with right middle cerebral artery (MCA) occlusion resulting in dense left hemiplegia and mutism and review of the literature. Intervention(s). Mechanical thrombectomy, intra-arterial administration of rt-PA, vasodilators, and platelet inhibitors, and systemic anticoagulation and subsequent critical care support. Results. Restoration of right MCA blood flow and complete resolution of neurologic deficits. Conclusion. We report the gratifying outcome of treatment of a case of AIS in a pediatric patient treated with hyperacute therapies geared to arterial recanalization and subsequent neurologic critical care and review the pertinent literature. Guidelines for the emergency room management of pediatric AIS from prospective, randomized trials are needed