Graph 4-braid groups and Massey products

We first show that the braid group over a graph topologically containing no $\Theta$-shape subgraph has a presentation related only by commutators. Then using discrete Morse theory and triple Massey products, we prove that a graph topologically contains none of four prescribed graphs if and only if its 4-braid groups is a right-angled Artin group.Comment: 23 pages, 4 figure

The Mechanical Impact of the Tibetan Plateau on the Seasonal Evolution of the South Asian Monsoon

The impact of the Tibetan Plateau on the South Asian monsoon is examined using a hierarchy of atmospheric general circulation models. During the premonsoon season and monsoon onset (April–June), when westerly winds over the Southern Tibetan Plateau are still strong, the Tibetan Plateau triggers early monsoon rainfall downstream, particularly over the Bay of Bengal and South China. The downstream moist convection is accompanied by strong monsoonal low-level winds. In experiments where the Tibetan Plateau is removed, monsoon onset occurs about a month later, but the monsoon circulation becomes progressively stronger and reaches comparable strength during the mature phase. During the mature and decaying phase of monsoon (July–September), when westerly winds over the Southern Tibetan Plateau almost disappear, monsoon circulation strength is not much affected by the presence of the Tibetan Plateau. A dry dynamical core with east–west-oriented narrow mountains in the subtropics consistently simulates downstream convergence with background zonal westerlies over the mountain. In a moist atmosphere, the mechanically driven downstream convergence is expected to be associated with significant moisture convergence. The authors speculate that the mechanically driven downstream convergence in the presence of the Tibetan Plateau is responsible for zonally asymmetric monsoon onset, particularly over the Bay of Bengal and South China

Comments on "The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess" - Reply

We thank Chang and Lin for their thoughtful and constructive comments on our study (Park et al. 2010). In Park et al. (2010), we did not explicitly state that the topography-forced stationary waves are the direct cause for the reduced downstream transient eddy kinetic energy (EKE). The response of stationary waves to topography may saturate even with a relatively small mountain (Cook and Held 1992); furthermore, their magnitudes are much smaller than thermally forced stationary waves (Chang 2009; Held et al. 2002). Instead, we suggest that quasistationary waves generated by the central Asian mountains may strongly affect North Pacific storminess by changing the year-to-year variability of westerly winds over the eastern Eurasian continent. Observational analyses indicate that the midwinter suppression of North Pacific storminess does not occur every year. Some years experience stronger and more meridionally confined zonal winds over the western North Pacific, leading to stronger midwinter suppression (Harnik and Chang 2004; Nakamura and Sampe 2002)

It is broken and accented : Non-native English-speaking (NNES) Graduate Students\u27 Perceptions toward NNES Instructors\u27 English

This study investigates the perceptions of non-native English-speaking graduate students towards non-native English speaking (NNES) instructors’ accented English. Students (N=161) who were enrolled in an oral English course at Purdue University participated in a survey. Follow-up interviews were conducted with voluntary participants (N=9) to examine the perceptions of NNES graduate students towards NNES instructors in depth. The findings in the survey showed that more than one third of the participants experienced difficulty with their NNES instructors due to their limited intelligibility and restricted command of English. Furthermore, one third of the participants expressed that they would transfer to another section of a course if the NNES instructor of the course speaks highly accented English. However, the majority of them believed NNES instructors can be as effective as NNS instructors. More overtly negative views were found during the interviews; many of the interviewees revealed strong desire to avoid NNES instructors with particular language backgrounds. Familiarity with the accents also played a significant role in ameliorating their negative perceptions toward NNES instructors. When there were communication breakdowns between the respondents and their NNES instructors, they tended to give up listening to the lectures and sought other resources or solutions to keep up their study address difficulties. Moreover, the majority of the interview participants expressed that they would avoid to discuss discussing the communication issues with their NNES instructors directly as it would be seen as rude and disrespectful

Modeling and Simulation of Acoustic Pressure Field for Ultrasonic Tactile Displays

As the virtual and augmented reality industry continues to grow, it is important to develop a tactile display technology that can seamlessly integrate into a multimodal VR experience. Ultrasonic haptic display technology uses a phased array of ultrasound transducers to create a mid-air pressure focal point, and a modulation of this radiation field at a frequency around 100-300 Hz can stimulate the mechanoreceptors in the skin to produce a tactile sensation. Optimizing this technology to create a strong pressure intensity and focality at low cost and in small space can help open up a new commercial market for tactile displays.This study explores the creation of a simple and modularized pressure field simulator for ultrasonic haptic displays using a simplified model of transducer radiation pattern. The radiation behavior is broken down to a combination of an on-axis radiation behavior and a directivity behavior, each modeled by an exponential and a Gaussian function, respectively. Then, some physical characteristics of phased array are examined to evaluate their influence on peak intensity of focal peak, focal radius, and number of significant secondary focal peaks. The results of the simulator are then compared against the real pressure field of a haptic display prototype

Formation Dynamics of Carbon Atomic Chain from Graphene by Electron Beam Irradiation

Carbon has numerous allotropes and various crystalline forms with full dimensionalities such as diamond, graphite, fullerenes, and carbon nanotubes leading a wide range of applications. Since the emerge of graphene consisting of a single atomic layer of carbon atoms, a fabrication of all-carbon-based device with combination of one-, two-, and three-dimensional carbons has become a hot issue. Here, we introduce an ultimate one-dimensional carbon atomic chain. Carbon atomic chains were experimentally created by removing atoms from monolayer graphene sheet under electron beam inside transmission electron microscope (TEM). A series of TEM images demonstrate the dynamics of carbon atomic chains over time from the formation, transformation, and then breakage