569 research outputs found
Weakening Assumptions for Deterministic Subexponential Time Non-Singular Matrix Completion
In (Kabanets, Impagliazzo, 2004) it is shown how to decide the circuit
polynomial identity testing problem (CPIT) in deterministic subexponential
time, assuming hardness of some explicit multilinear polynomial family for
arithmetical circuits. In this paper, a special case of CPIT is considered,
namely low-degree non-singular matrix completion (NSMC). For this subclass of
problems it is shown how to obtain the same deterministic time bound, using a
weaker assumption in terms of determinantal complexity.
Hardness-randomness tradeoffs will also be shown in the converse direction,
in an effort to make progress on Valiant's VP versus VNP problem. To separate
VP and VNP, it is known to be sufficient to prove that the determinantal
complexity of the m-by-m permanent is . In this paper it is
shown, for an appropriate notion of explicitness, that the existence of an
explicit multilinear polynomial family with determinantal complexity
m^{\omega(\log m)}G_nO(n^{1/\sqrt{\log n}})G_nM(x)poly(n)ndet(M(x))$ is a multilinear polynomial
Deterministic Black-Box Identity Testing -Ordered Algebraic Branching Programs
In this paper we study algebraic branching programs (ABPs) with restrictions
on the order and the number of reads of variables in the program. Given a
permutation of variables, for a -ordered ABP (-OABP), for
any directed path from source to sink, a variable can appear at most once
on , and the order in which variables appear on must respect . An
ABP is said to be of read , if any variable appears at most times in
. Our main result pertains to the identity testing problem. Over any field
and in the black-box model, i.e. given only query access to the polynomial,
we have the following result: read -OABP computable polynomials can be
tested in \DTIME[2^{O(r\log r \cdot \log^2 n \log\log n)}].
Our next set of results investigates the computational limitations of OABPs.
It is shown that any OABP computing the determinant or permanent requires size
and read . We give a multilinear polynomial
in variables over some specifically selected field , such that
any OABP computing must read some variable at least times. We show
that the elementary symmetric polynomial of degree in variables can be
computed by a size read OABP, but not by a read OABP, for
any . Finally, we give an example of a polynomial and two
variables orders , such that can be computed by a read-once
-OABP, but where any -OABP computing must read some variable at
least $2^n
Beirut Blast: A port city in crisis
On 4th of August 2020, the Lebanese capital and port city, Beirut, was rocked by a massive explosion that has killed hundreds and injured thousands more, ravaging the heart of the city’s nearby downtown business district and neighbouring housing areas, where more than 750,000 people live. The waterfront neighbourhood and a number of dense residential neighbourhoods in the city’s eastern part were essentially flattened. Lebanese Government officials believe that the blast was caused by around 2,700 tonnes of ammonium nitrate stored near the city’s cargo port without proper control for six years.
The disaster devastating Beirut’s port and city shows the latent danger of safe storage of potentially dangerous goods in modern ports, particularly ones located close to the heart of the city. The huge blast tore through major grain silos, stoking fears of shortages in a nation that imports nearly all its food and is already reeling from economic crisis. As the WFP (World Food Programme) said in a statement: the blast will “exacerbate the grim economic and food-security situation.” The Beirut blast also reminds us of the importance of ports in the contemporary globalized world. It calls our attention to safety and security, of governance and collaboration between port and city or region and of accessibility to the hinterland. What do local governments and port cities need to do to enhance the safety and security issues in port terminals? Or, to put it differently, how do we reconcile this challenge between the ports we need to feed us, serve us, provide us with medicines, equipment, etc. and the ports that threatens us
Port-City Transition: Past and Emerging Socio-Spatial Imaginaries and Uses in Rotterdam's Makers District
This article explores old and emerging socio-spatial imaginaries and uses of Rotterdam's Makers District. The district comprises two urban harbors - Merwe Vierhavens and Rotterdamsche Droogdok Maatschappij - historically in use as bustling trade, storage, and ship yarding nodes of the city’s port activities. At the turn of the millennium, technological advancements made it possible to move many port-related activities out of the area and farther out of the city, gradually hollowing out these harbors’ port-related economic foundations and opening opportunities for new uses and imaginaries. This article traces the transition by detailing how the boundary between the city and the port has become more porous in this district. It does so by offering original empirical evidence on the flows of users in and out of the area in recent years, based on location quotients, while also applying a content analysis of the profiles of companies and institutions currently inhabiting and working in these transformed port-city spaces. On the one hand, the results show how the ongoing port-city transition in Rotterdam's Makers District combines carefully curated interventions and infrastructure plans seeking to progressively adapt the area to new purposes, while maintaining some of its former functions. On the other hand, they highlight the pioneering role of more bottom-up initiatives and innovative urban concepts, springing from the creative industries and maker movement. The article offers insights into the emerging uses and imaginaries attached to the district, while also showing the resilience and adaptation of port legacies
Exploring the conditions for inclusive port development: the case of Indonesia
Governments around the world are adopting inclusive growth agendas. The ambition to align economic growth ambitions with broader-based social benefits is increasingly embraced by corporations to limit the ‘negative externalities’ and enhance the ‘positive externalities’ of their operations. Therefore, micro-level corporate strategies and macro-level national ambitions meet at the meso-level of networks and clusters. This requires societal spheres to collaborate and search for alternative governance constellations. In this discourse, port development is only recently receiving attention. In March 2018, ports around the world signed the World Ports Sustainability Program declaration, which aims to contribute to the sustainable development goals (SDGs), whilst a number of national port (master) plans have started to include social along with environmental standards. Extant studies on partnering and stakeholder inclusion in port development are proliferating but are primarily aimed at environmental rather than social (inclusion) issues. This paper adopts an exploratory research design to consider conditions for inclusive port development. A novel taxonomy considers port development as a driver for inclusive growth, where partnerships are the missing link between micro-level business strategies and macro-level effects in the port region and economy at large. This paper shows the first findings and delineates areas for further research
Prospects for high-resolution microwave spectroscopy of methanol in a Stark-deflected molecular beam
Recently, the extremely sensitive torsion-rotation transitions in methanol
have been used to set a tight constraint on a possible variation of the
proton-to-electron mass ratio over cosmological time scales. In order to
improve this constraint, laboratory data of increased accuracy will be
required. Here, we explore the possibility for performing high-resolution
spectroscopy on methanol in a Stark-deflected molecular beam. We have
calculated the Stark shift of the lower rotational levels in the ground
torsion-vibrational state of CH3OH and CD3OH molecules, and have used this to
simulate trajectories through a typical molecular beam resonance setup.
Furthermore, we have determined the efficiency of non-resonant multi-photon
ionization of methanol molecules using a femtosecond laser pulse. The described
setup is in principle suited to measure microwave transitions in CH3OH at an
accuracy below 10^{-8}
Using fluorescent and non-fluorescent Sterols to Study Rafts and Intracellular Cholesterol Transport in Mammalian Cells
Cholesterol is an essential component in the membranes of most eukaryotic cells, in which it mediates many functions including membrane fluidity, permeability and the formation of ordered membrane domains. In this work a fluorescent and a non-fluorescent cholesterol analog were characterized as tools to study cholesterol. Next, these analogs were used to study two specific cell biological processes that involve cholesterol, i.e. the structure and function of ordered membrane domains/rafts and intracellular cholesterol transport.
The most common method for studying ordered membrane domains is by disrupting them by cholesterol depletion. Because cholesterol depletion affects many cellular functions besides those mediated by membrane domains, this procedure is highly unspecific. The cellular exchange of cholesterol by desmosterol as a tool to study ordered membrane domains was characterized. It turned out that the ability of desmosterol to form and stabilize membrane domains in vitro was weaker compared to cholesterol. This result was reinforced by atomistic scale simulations that indicated that desmosterol has a lower ordering effect on phospholipid acyl chains. Three procedures were established for exchanging cellular cholesterol by desmosterol. In cells in which desmosterol was the main sterol, insulin signaling was attenuated. The results suggest that this was caused by desmosterol destabilizing membrane rafts. Contrary to its effect on ordered membrane domains it was found that replacing cholesterol by desmosterol does not change cell growth/viability, subcellular sterol distribution, Golgi integrity, secretory pathway, phospholipid composition and membrane fluidity. Together these results suggest that exchanging cellular cholesterol by desmosterol provides a selective tool for perturbing rafts. Next, the importance of cholesterol for the structure and function of caveolae was analyzed by exchanging the cellular cholesterol by desmosterol. The sterol exchange reduced the stability of caveolae as determined by detergent resistance of caveolin-1 and heat resistance of caveolin-1 oligomers. Also the sterol exchange led to aberrations in the caveolar structure; the morphology of caveolae was altered and there was a larger variation in the amount of caveolin-1 molecules per caveola. These results demonstrate that cholesterol is important for caveolar stability and structural homogeneity.
In the second part of this work a fluorescent cholesterol analog was characterized as a tool to study cholesterol transport. Tight control of the intracellular cholesterol distribution is essential for many cellular processes. An important mechanism by which cells regulate their membrane cholesterol content is by cholesterol traffic, mostly from the plasma membrane to lipid droplets. The fluorescent sterol probe BODIPY-cholesterol was characterized as a tool to analyze cholesterol transport between the plasma membrane, the endoplasmic reticulum (ER) and lipid droplets. The behavior of BODIPY-cholesterol was compared to that of natural sterols, using both biochemical and live-cell microcopy assays. The results show that the transport kinetics of BODIPY-cholesterol between the plasma membrane, the ER and lipid droplets is similar to that of unesterified cholesterol. Next, BODIPY-cholesterol was utilized to analyze the importance of oxysterol binding protein related proteins (ORPs) for cholesterol transport between the plasma membrane, the ER, and lipid droplets in mammalian cells. By overexpressing all human ORPs it turned out that especially ORP1S and ORP2 enhanced sterol transport from the plasma membrane to lipid droplets. Our results suggest that the increased sterol transport takes place between the plasma membrane and ER and not between the ER and lipid droplets. Simultaneous knockdown of ORP1S and ORP2 resulted in a moderate but significant inhibition of sterol traffic from the plasma membrane to ER and lipid droplets, suggesting a physiological role for these ORPs in this process. The two phenylalanines in an acidic tract (FFAT) motif in ORPs, which mediates interaction with vesicle associated membrane protein associated proteins (VAPs) in the ER, was not necessary for mediating sterol transport. However, VAP silencing slowed down sterol transport, most likely by destabilizing ORPs containing a FFAT motif
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