arXiv subjects

Kevin Sun

Publications and source records attributed to Kevin Sun.

6 recordsLinked to original sources

arXiv2023

Spatially-coded Fourier ptychography: flexible and detachable coded thin films for quantitative phase imaging with uniform phase transfer characteristics

Fourier ptychography (FP) is an enabling imaging technique that produces high-resolution complex-valued images with extended field coverages. However, when FP images a phase object with any specific spatial frequency, the captured images contain only constant values, rendering the recovery of the corresponding linear phase ramp impossible. This challenge is not unique to FP but also affects other common microscopy techniques -- a rather counterintuitive outcome given their widespread use in phase imaging. The underlying issue originates from the non-uniform phase transfer characteristic inherent in microscope systems, which impedes the conversion of object wavefields into discernible intensity variations. To address this challenge, we present spatially-coded Fourier ptychography (scFP), a new method that synergizes FP with spatial-domain coded detection for true quantitative phase imaging. In scFP, a flexible and detachable coded thin film is attached atop the image sensor in a regular FP setup. The spatial modulation of this thin film ensures a uniform phase response across the entire synthetic bandwidth. It improves reconstruction quality and corrects refractive index underestimation issues prevalent in conventional FP and related tomographic implementations. The inclusion of the coded thin film further adds a new dimension of measurement diversity in the spatial domain. The development of scFP is expected to catalyse new research directions and applications for phase imaging, emphasizing the need for true quantitative accuracy with uniform frequency response.

physics.optics↗

arXiv2022

Historical Patterns and Recent Impacts of Chinese Investors in United States Real Estate

Since supplanting Canada in 2014, Chinese investors have been the lead foreign buyers of U.S. real estate, concentrating their purchases in urban areas with higher Chinese populations like California. The reasons for investment include prestige, freedom from capital confiscation, and safe, diversified opportunities from abroad simply being more lucrative and available than in their home country, where the market is eroding. Interestingly, since 2019, Chinese investors have sold a net 23.6 billion dollars of U.S. commercial real estate, a stark contrast to past acquisitions between 2013 to 2018 where they were net buyers of almost 52 billion dollars worth of properties. A similar trend appears in the residential real estate segment too. In both 2017 and 2018, Chinese buyers purchased over 40,000 U.S. residential properties which were halved in 2019 and steadily declined to only 6,700 in the past year. This turnaround in Chinese investment can be attributed to a deteriorating relationship between the U.S. and China during the Trump Presidency, financial distress in China, and new Chinese government regulations prohibiting outbound investments. Additionally, while Chinese investment is a small share of U.S. real estate (~1.5% at its peak), it has outsized impacts on market valuations of home prices in U.S. zip codes with higher populations of foreign-born Chinese, increasing property prices and exacerbating the issue of housing affordability in these areas. This paper investigates the rapid growth and decline of Chinese investment in U.S. real estate and its effect on U.S. home prices in certain demographics.

econ.GN↗

arXiv2021

Approximations for Allocating Indivisible Items with Concave-Additive Valuations

We study a general allocation setting where agent valuations are concave additive. In this model, a collection of items must be uniquely distributed among a set of agents, where each agent-item pair has a specified utility. The objective is to maximize the sum of agent valuations, each of which is an arbitrary non-decreasing concave function of the agent's total additive utility. This setting was studied by Devanur and Jain (STOC 2012) in the online setting for divisible items. In this paper, we obtain both tight multiplicative and additive approximations in the offline setting for indivisible items. Our approximations depend on novel parameters that measure the local multiplicative/additive curvatures of each agent valuation, which we show correspond directly to the integrality gap of the natural assignment convex program of the problem. Furthermore, we extend our additive guarantees to obtain constant multiplicative approximations for Asymmetric Nash Welfare Maximization when agents have smooth valuations (Fain et al. EC 2018, Fluschnik et al. AAAI 2019). This algorithm also yields an interesting tatonnement-style interpretation, where agents adjust uniform prices and items are assigned according to maximum weighted bang-per-buck ratios.

cs.DS↗

arXiv2020

Online Algorithms for Weighted Paging with Predictions

In this paper, we initiate the study of the weighted paging problem with predictions. This continues the recent line of work in online algorithms with predictions, particularly that of Lykouris and Vassilvitski (ICML 2018) and Rohatgi (SODA 2020) on unweighted paging with predictions. We show that unlike unweighted paging, neither a fixed lookahead nor knowledge of the next request for every page is sufficient information for an algorithm to overcome existing lower bounds in weighted paging. However, a combination of the two, which we call the strong per request prediction (SPRP) model, suffices to give a 2-competitive algorithm. We also explore the question of gracefully degrading algorithms with increasing prediction error, and give both upper and lower bounds for a set of natural measures of prediction error.

cs.DS↗

arXiv2020

Sparsification of Directed Graphs via Cut Balance

In this paper, we consider the problem of designing cut sparsifiers and sketches for directed graphs. To bypass known lower bounds, we allow the sparsifier/sketch to depend on the balance of the input graph, which smoothly interpolates between undirected and directed graphs. We give nearly matching upper and lower bounds for both for-all (cf. Bencz\'ur and Karger, STOC 1996) and for-each (Andoni et al., ITCS 2016) cut sparsifiers/sketches as a function of cut balance, defined the maximum ratio of the cut value in the two directions of a directed graph (Ene et al., STOC 2016). We also show an interesting application of digraph sparsification via cut balance by using it to give a very short proof of a celebrated maximum flow result of Karger and Levine (STOC 2002).

cs.DS↗

arXiv2017

Select and Permute: An Improved Online Framework for Scheduling to Minimize Weighted Completion Time

In this paper, we introduce a new online scheduling framework for minimizing total weighted completion time in a general setting. The framework is inspired by the work of Hall et al. [Mathematics of Operations Research, Vol 22(3):513-544, 1997] and Garg et al. [Proc. of Foundations of Software Technology and Theoretical Computer Science, pp. 96-107, 2007], who show how to convert an offline approximation to an online scheme. Our framework uses two offline approximation algorithms (one for the simpler problem of scheduling without release times, and another for the minimum unscheduled weight problem) to create an online algorithm with provably good competitive ratios. We illustrate multiple applications of this method that yield improved competitive ratios. Our framework gives algorithms with the best or only known competitive ratios for the concurrent open shop, coflow, and concurrent cluster models. We also introduce a randomized variant of our framework based on the ideas of Chakrabarti et al. [Proc. of International Colloquium on Automata, Languages, and Programming, pp. 646-657, 1996] and use it to achieve improved competitive ratios for these same problems.

cs.DS↗