Determining the local dark matter density with LAMOST data

Measurement of the local dark matter density plays an important role in both Galactic dynamics and dark matter direct detection experiments. However, the estimated values from previous works are far from agreeing with each other. In this work, we provide a well-defined observed sample with 1427 G \& K type main-sequence stars from the LAMOST spectroscopic survey, taking into account selection effects, volume completeness, and the stellar populations. We apply a vertical Jeans equation method containing a single exponential stellar disk, a razor thin gas disk, and a constant dark matter density distribution to the sample, and obtain a total surface mass density of $\rm {78.7 ^{+3.9}_{-4.7}\ M_{\odot}\ pc^{-2}}$up to 1 kpc and a local dark matter density of$0.0159^{+0.0047}_{-0.0057}\,\rm M_{\odot}\,\rm pc^{-3}$. We find that the sampling density (i.e. number of stars per unit volume) of the spectroscopic data contributes to about two-thirds of the uncertainty in the estimated values. We discuss the effect of the tilt term in the Jeans equation and find it has little impact on our measurement. Other issues, such as a non-equilibrium component due to perturbations and contamination by the thick disk population, are also discussed.Comment: 11 pages, 10 figure

Fraction magnitude understanding and its unique role in predicting general mathematics achievement at two early stages of fraction instruction

Background Prior studies on fraction magnitude understanding focused mainly on students with relatively sufficient formal instruction on fractions whose fraction magnitude understanding is relatively mature. Aim This study fills a research gap by investigating fraction magnitude understanding in the early stages of fraction instruction. It extends previous findings to children with limited and primary formal fraction instruction. Sample(s) Thirty‐five fourth graders with limited fraction instruction and forty fourth graders with primary fraction instruction were recruited from a Chinese primary school. Methods Children's fraction magnitude understanding was assessed with a fraction number line estimation task. Approximate number system (ANS) acuity was assessed with a dot discrimination task. Whole number knowledge was assessed with a whole number line estimation task. General reading and mathematics achievements were collected concurrently and 1 year later. Results In children with limited fraction instruction, fraction representation was linear and fraction magnitude understanding was concurrently related to both ANS and whole number knowledge. In children with primary fraction instruction, fraction magnitude understanding appeared to (marginally) significantly predict general mathematics achievement 1 year later. Conclusions Fraction magnitude understanding emerged early during formal instruction of fractions. ANS and whole number knowledge were related to fraction magnitude understanding when children first began to learn about fractions in school. The predictive value of fraction magnitude understanding is likely constrained by its sophistication level

Essays on housing economics and real estate finance

This dissertation contains three chapters on topics in the field of housing economics and real estate finance. This first chapter examines the impact of mortgage credit supply contraction on the supply and pricing of rental housing in the wake of the 2008-2009 financial crisis. We provide estimates of (1) the size of mortgage credit supply shocks at the county-level across the United States; (2) the impact of mortgage credit supply shock on housing rents and housing supply. To estimate the size of the credit supply shock, we use non-parametric methods to identify lender specific supply-side shocks; we then use a shift-share approach to aggregate a measure of credit supply shock at county level. Using the county-level credit supply shock as instruments for changes in mortgage denial rates, we reveal that housing rents respond positively to the contraction of the mortgage supply. The impact on housing rents is heterogeneous among different income groups. Moreover, we also document that the contraction of the mortgage supply resulted in a decline in home sales, and increased the share of renter-occupied units. Moreover, households in the lowest income groups are most affected. The finding suggests that recent regulatory changes in mortgage lending may have unintended consequences, leading to reduced home ownership and increased rental housing prices for the lowest income households. The second chapter provides a comprehensive investigations into the heterogeneity of home improvement and maintenance activities that has not been well explored by literature. Using American Housing Survey data on home improvement and maintenance expenditure, we obtain the following empirical evidence: First, speculating buyers spend significantly higher amount on home improvement compared with non-speculating buyers, and are more likely to perform major improvement. Second, households spend significantly more following the home purchase rather than prior to resale. This could be attributed to search friction that prevent households from matching to the perfect home. Third, the spending of home improvement and maintenance exhibits considerable variation across housing segments. In addition, there is considerable regional variation. The third chapter studies housing demand inspired by the fact that housing’s relative price, share of expenditure, and “unaffordability” have all grown since 1970. We estimate housing demand using a novel compensated framework over space and an uncompensated framework over time. Our specifications pass tests imposed by rationality and household mobility. Housing demand is income and price inelastic, and appears to fall with household size. We provide a numerical non-homothetic constant elasticity of substitution utility function for improved quantitative modeling. An ideal cost-of-living index demonstrates that the poor have been disproportionately impacted by rising relative rents, which have greatly amplified increases in real income inequality

Wave attenuation and focusing by a parabolic arc pontoon breakwater

In the multifunctional system consisting of point absorber wave energy converters and a pontoon breakwater, the breakwater plays an essential role in attenuating waves on the lee side and amplifying waves for a better energy harvesting on the stoss side. The structure of breakwater is expectedly improved to enhance its wave attenuation and amplification at the same time. Here we present a novel parabolic arc breakwater and show that for a range of typical regular incident waves, it can attenuate more wave elevation and focus high waves in several regions in comparison to a straight breakwater. In further frequency-domain investigations, a special relatively-low frequency associated with the parabolic arc breakwater configuration is found and named as the critical frequency, closed to which splendid attenuation and focusing performance can be achieved. A systematic parametric study on the geometric factors (draft, width, and chord length) of the parabolic arc breakwater is thereafter carried out to examine their influence on the attenuation and focusing performance at the critical frequency. We find that an increase of the draft can reduce the critical frequency greatly so as to let it be within the real sea states, meanwhile slightly affecting the attenuation performance. An increase of the chord length has an uncertain but not large influence on the attenuation performance, whereas it enhances substantially the focusing performance. Simultaneously, an amplification rate up to 3.06 in two relatively-large focal areas in a prescribed deployment zone and an average attenuation rate of approximately 68% in a prescribed protection zone could be obtained in a commonly observed coastal wave.</p

Wave attenuation and focusing by a parabolic arc pontoon breakwater

In the multifunctional system consisting of point absorber wave energy converters and a pontoon breakwater, the breakwater plays an essential role in attenuating waves on the lee side and amplifying waves for a better energy harvesting on the stoss side. The structure of breakwater is expectedly improved to enhance its wave attenuation and amplification at the same time. Here we present a novel parabolic arc breakwater and show that for a range of typical regular incident waves, it can attenuate more wave elevation and focus high waves in several regions in comparison to a straight breakwater. In further frequency-domain investigations, a special relatively-low frequency associated with the parabolic arc breakwater configuration is found and named as the critical frequency, closed to which splendid attenuation and focusing performance can be achieved. A systematic parametric study on the geometric factors (draft, width, and chord length) of the parabolic arc breakwater is thereafter carried out to examine their influence on the attenuation and focusing performance at the critical frequency. We find that an increase of the draft can reduce the critical frequency greatly so as to let it be within the real sea states, meanwhile slightly affecting the attenuation performance. An increase of the chord length has an uncertain but not large influence on the attenuation performance, whereas it enhances substantially the focusing performance. Simultaneously, an amplification rate up to 3.06 in two relatively-large focal areas in a prescribed deployment zone and an average attenuation rate of approximately 68% in a prescribed protection zone could be obtained in a commonly observed coastal wave.</p

A Non-Contact PCB Multi-Fault Diagnosis Algorithm Based on Scalar Magnetic Field Fusion Feature and Transformer Architecture

Non-contact printed circuit board (PCB) fault diagnosis has been widely applied in PCB detection and maintenance. Due to objective factors such as visual blind spots, low-loss circuit structure design, and frequency insensitivity, traditional algorithms based on visual and temperature features are limited in practice. Therefore, the algorithm based on electromagnetic features containing rich physical connotations and prominent frequency features has received attention in PCB fault diagnosis. Based on the basic principles of electromagnetic physics and PCB fault relationship, this article proposes a scalar magnetic field source feature and further improves feature performance by adding topological relationships of multi-faults to generate the fusion feature. The backbone of the PCB diagnosis model is established on the Transformer architecture, effectively utilizing self-attention and parallel computing mechanisms to explore the inner correlation between each group feature. The paper provides a new non-contact PCB fault diagnosis solution that enriches existing methods. Besides, through actual experiments setting up multi-fault PCBs, the feasibility of our process is proved based on the proposed features and models. The specific multiple indicators Overall Precision (OP), Per Class Precision (CP), Overall Recall (OR), Per Class Recall (CR), Overall F1 Measure (OF1), Per Class F1 Measure (CF1), Accuracy (ACC), Mean Average Precision (mAP) are 98.55%, 94.89%, 98.55%, 95.11%, 98.55%, 95.32%, 96.01%, and 97.27

On hydrodynamic characteristics of gap resonance between two fixed bodies in close proximity

The resonant water motion inside a narrow gap between two identical fixed boxes that are in side-by-side configuration is investigated using a two-dimensional (2D) numerical wave tank based on OpenFOAM®, an open source CFD package. Gap resonance is excited by regular waves with various wave heights, ranging from linear waves to strong nonlinear waves. This paper mainly focuses on the harmonic analyses of the free-surface elevation in the narrow gap and wave loads (including the horizontal wave forces, the vertical wave forces and the moments) on the bodies. It is found that the influences of the incident wave height on the higher-order harmonic components of different physical quantities are quite different. The effects of the incident wave height on the reflection, transmission and energy loss coefficients are also discussed. Finally, aiming at the quantitative estimation of the response time and the damping time of gap resonance, two different methods are proposed and verified for the first time on gap resonance.</p