678 research outputs found
Dimension and cut vertices: an application of Ramsey theory
Motivated by quite recent research involving the relationship between the
dimension of a poset and graph-theoretic properties of its cover graph, we show
that for every , if is a poset and the dimension of a subposet
of is at most whenever the cover graph of is a block of the cover
graph of , then the dimension of is at most . We also construct
examples which show that this inequality is best possible. We consider the
proof of the upper bound to be fairly elegant and relatively compact. However,
we know of no simple proof for the lower bound, and our argument requires a
powerful tool known as the Product Ramsey Theorem. As a consequence, our
constructions involve posets of enormous size.Comment: Final published version with updated reference
Power system stability scanning and security assessment using machine learning
Future grids planning requires a major departure from conventional power system planning, where only a handful of the most critical scenarios is analyzed. To account for a wide range of possible future evolutions, scenario analysis has been proposed in many industries. As opposed to the conventional power system planning, where the aim is to find an optimal transmission and/or generation expansion plan for an existing grid, the aim in future grids scenario analysis is to analyze possible evolution pathways to inform power system planning and policy making. Therefore, future grids’ planning may involve large amount of scenarios and the existing planning tools may no longer suitable. Other than the raised future grids’ planning issues, operation of future grids using conventional tools is also challenged by the new features of future grids such as intermittent generation, demand response and fast responding power electronic plants which lead to much more diverse operation conditions compared to the existing networks. Among all operation issues, monitoring stability as well as security of a power system and action with deliberated preventive or remedial adjustment is of vital important. On- line Dynamic Security Assessment (DSA) can evaluate security of a power system almost instantly when current or imminent operation conditions are supplied. The focus of this dissertation are, for future grid planning, to develop a framework using Machine Learning (ML) to effectively assess the security of future grids by analyzing a large amount of the scenarios; for future grids operation, to propose approaches to address technique issues brought by future grids’ diverse operation conditions using ML techniques. Unsupervised learning, supervised learning and semi-supervised learning techniques are utilized in a set of proposed planning and operation security assessment tools
Essays on Financial Development in China : Political Connection, Fiscal Effect and Openness
早大学位記番号:新6790早稲田大
High dynamic range display systems
High contrast ratio (CR) enables a display system to faithfully reproduce the real objects. However, achieving high contrast, especially high ambient contrast (ACR), is a challenging task. In this dissertation, two display systems with high CR are discussed: high ACR augmented reality (AR) display and high dynamic range (HDR) display. For an AR display, we improved its ACR by incorporating a tunable transmittance liquid crystal (LC) film. The film has high tunable transmittance range, fast response time, and is fail-safe. To reduce the weight and size of a display system, we proposed a functional reflective polarizer, which can also help people with color vision deficiency. As for the HDR display, we improved all three aspects of the hardware requirements: contrast ratio, color gamut and bit-depth. By stacking two liquid crystal display (LCD) panels together, we have achieved CR over one million to one, 14-bit depth with 5V operation voltage, and pixel-by-pixel local dimming. To widen color gamut, both photoluminescent and electroluminescent quantum dots (QDs) have been investigated. Our analysis shows that with QD approach, it is possible to achieve over 90% of the Rec. 2020 color gamut for a HDR display. Another goal of an HDR display is to achieve the 12-bit perceptual quantizer (PQ) curve covering from 0 to 10,000 nits. Our experimental results indicate that this is difficult with a single LCD panel because of the sluggish response time. To overcome this challenge, we proposed a method to drive the light emitting diode (LED) backlight and the LCD panel simultaneously. Besides relatively fast response time, this approach can also mitigate the imaging noise. Finally yet importantly, we improved the display pipeline by using a HDR gamut mapping approach to display HDR contents adaptively based on display specifications. A psychophysical experiment was conducted to determine the display requirements
On the dimension of posets with cover graphs of treewidth
In 1977, Trotter and Moore proved that a poset has dimension at most
whenever its cover graph is a forest, or equivalently, has treewidth at most
. On the other hand, a well-known construction of Kelly shows that there are
posets of arbitrarily large dimension whose cover graphs have treewidth . In
this paper we focus on the boundary case of treewidth . It was recently
shown that the dimension is bounded if the cover graph is outerplanar (Felsner,
Trotter, and Wiechert) or if it has pathwidth (Bir\'o, Keller, and Young).
This can be interpreted as evidence that the dimension should be bounded more
generally when the cover graph has treewidth . We show that it is indeed the
case: Every such poset has dimension at most .Comment: v4: minor changes made following helpful comments by the referee
Critical study of parallel programming frameworks for distributed applications
Parallel programming frameworks such as the Message Passing Interface (MPI), Partitioned Global Address Space (PGAS) languages, Charm++, Legion and High Performance Parallel X (HPX) have been used in several scientific domains -- such as bioinformatics, physics, chemistry, and others -- to implement distributed applications. These frameworks allow distributing data and computation across the different nodes (or machines) of a high-performance computing cluster. However, these frameworks differ in their programmability, performance, and suitability to different cluster settings. For example, some of these frameworks have been designed to support applications running on homogeneous clusters that include only general purpose CPUs, while others offer support for heterogeneous clusters that include accelerators, such as graphics processing units (GPUs). Hence, it is important for programmers to select the programing framework that is best suited to the characteristics of their application (i.e. its computation and communication patterns) and the hardware setup of the target high-performance computing cluster
Dimension of posets with planar cover graphs excluding two long incomparable chains
It has been known for more than 40 years that there are posets with planar
cover graphs and arbitrarily large dimension. Recently, Streib and Trotter
proved that such posets must have large height. In fact, all known
constructions of such posets have two large disjoint chains with all points in
one chain incomparable with all points in the other. Gutowski and Krawczyk
conjectured that this feature is necessary. More formally, they conjectured
that for every , there is a constant such that if is a poset
with a planar cover graph and excludes , then
. We settle their conjecture in the affirmative. We also discuss
possibilities of generalizing the result by relaxing the condition that the
cover graph is planar.Comment: New section on connections with graph minors, small correction
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