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Discretizing probability distributions and stochastic processes



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Susceptible-Infected-Recovered (SIR) Dynamics of COVID-19 and Economic Impact Permalink


After returning from my UK trip in early March 2020, just a few days before the closure of the border, I got interested in COVID-19, like many of us. I played around with some models and I derived a system of differential equations that turned out to be identical to the Kermack & McKendrick (1927) susceptible-infected-recovered (SIR) model. I searched the literature and found the closed-form solution by Harko et al. (2014). At that time, little was known about COVID-19, so I decided to estimate the epidemiological parameters from data to predict the epidemic dynamics.

In mid March 2020, the state of California imposed lockdown, my kids’ school suddenly closed (teachers were not yet used to online teaching, so education was laissez faire to parents), tennis courts closed, and we could no longer eat at restaurants. I thought this was all wrong. I have a medical degree and knew about herd immunity, so just shutting down everything would simply delay the problem without solving it, while imposing significant economic costs to the society. So for the first time in my career as a researcher, I worked on a project not just driven by scientific curiosity but also by a sense of civic duty to inform the public and policy makers.

According to my analysis in the figure below, imposing an early (and necessarily temporary) lockdown would simply delay the spread of the epidemic without affecting the ultimate toll.

Early mitigation

However, if the government delays intervention until about 1% of the population is infected, it would significantly reduce the total cumulative number of infections due to the build up of herd immunity, as the following figure shows.

Early mitigation

The analysis was of course not perfect, but because I thought the problem was urgent, I submitted this paper to AER: Insights on March 26, 2020. I presented the paper at the UCSD Econometrics seminar on March 31, 2020, and the paper was included in the first issue of the working paper series Covid Economics. It was also featured in the April 2020 VoxEU and May 2020 Fortune articles. This paper is by far my most cited paper. Unfortunately, AER: Insights rejected my paper in May 2020, by which time so many things have changed and there was so much competition, so I gave up the project.

After this experience, I started to question the relevance of traditional economics research. I felt that the long review process at typical economics journals prevents researchers from working on pressing issues. Of course, one could write a sophisticated paper after the pandemic is long gone (instead of writing a preliminary paper in a few weeks), but what is the benefit to the society? I also felt that most COVID papers written by economists encouraged lockdown and sided with increased government control, perhaps because researchers wanted to play the good guy. Later, I published a more sophisticated paper in JET.

Visualizing the Contraction Mapping Theorem Permalink


After my 2022 ECMA paper got accepted in January 2022, I got burnt out. I stopped doing research and spent most of my time playing tennis. I became a captain of a USTA 7.0 mixed doubles team (I had just become 4.0), and my wife and I recruited strong players, organized practices, and advanced to sectionals twice. In one of the Southern California sectionals, we narrowly lost in the semifinal.

My coauthor Jim Rauch was the recruitment chair when I got hired at UCSD in 2013. He was also the department chair from 2013 to 2016, so I had a lot of interaction with him. When we chatted in June 2022, he mentioned he was interested in a project to animate the contraction mapping theorem to help build intuition. We knew it had zero career benefit but it sounded fun, so I wrote up some pedagogical material and Matlab codes and Jim made the video.

We tried to publish this in journals on economic education but we got desk-rejected each time and the paper became dormant. Later, I was asked to review a paper at Qeios. Because I had never heard of that journal, I thought it was a predatory journal, but upon inspection its business model seemed interesting: they publish anything, but reviews are open and (to prevent abuse) not anonymous. So we posted our paper there, and we are happy that our paper and video have been well received.

Convergence of value functions

‘Ergodicity Economics’ is Pseudoscience Permalink


In May 2023, I read something about Ergodicity Economics, and I thought it was a completely nonsense pseudoscience promoted by failed self-proclaimed physicists. Although it had zero career benefit, to contribute to the public good to prevent the spread of pseudoscience, I spent a few days writing this critique. The paper got desk-rejected from Physical Review Letters, Physical Review E, and Chaos and became dormant. After my pleasant experience at Qeios, I published the paper there.



Radii of the Inscribed and Escribed Spheres of a Simplex Permalink

Published in International Journal of Geometry, 2014

(Mathematics) High-dimensional generalization of the fact that the sum of the reciprocals of the radii of escribed circles of a triangle equals the reciprocal of the radius of the inscribed circle; obtained those results in 1998 when I was freshman.

👍The Equity Premium and the One Percent Permalink

Published in Review of Financial Studies, 2020

👍(Finance, Theory, Empirical) In general equilibrium model with heterogeneous risk aversion and/or beliefs, the wealth distribution predicts excess stock returns, which we confirm in data using estate tax rate change as instrument.

👍A Theory of the Saving Rate of the Rich Permalink

Published in Journal of Economic Theory, 2021

👍(Theory, Macro) Prove asymptotic linearity of policy functions when preferences are homothetic; show that asymptotic marginal propensities to consume can be zero, implying a large saving rate of the rich.

Tail Behavior of Stopped Lévy Processes with Markov Modulation Permalink

Published in Econometric Theory, 2022

This is a follow up paper of Beare & Toda (2022), where we characterize the tail behavior of Markov-modulated Lévy processes that are stopped at state-dependent Poisson rates. In 2018, Brendan gave the project to Won-Ki, who was his student. I joined Won-Ki’s dissertation committee to advise on this project, and we essentially translated the discrete-time results in Beare & Toda (2022) to continuous-time. The CARA-Huggett economy example in Section 4 was recycled from an earlier version of Beare & Toda (2022).

👍Optimal Epidemic Control in Equilibrium with Imperfect Testing and Enforcement Permalink

Published in Journal of Economic Theory, 2022

👍(Theory) Study a behavioral SIR model with imperfect testing and government enforcement and show that equilibrium action is approximately static efficient in the sense that the laissez faire equilibrium allocation is close to the optimal short-term lockdown policy, implying that short-term lockdown policies are redundant.

Capital and Labor Income Pareto Exponents across Time and Space Permalink

Published in Review of Income and Wealth, 2022

(Power law, Empirical) Estimate capital and labor income Pareto exponents across 475 country-year observations and document that capital income inequality is higher than labor income inequality (median Pareto exponents 1.46 and 3.35 respectively) and the two inequalities are uncorrelated, suggesting importance of distinguishing the two.

👍Bubble Economics Permalink

Published in Journal of Mathematical Economics, 2024

👍(Theory, Macro, Finance) Self-contained review of the theory of asset price bubbles.

On Equilibrium Determinacy in Overlapping Generations Models with Money Permalink

Published in Economics Letters, 2024

I started to study models of bubble and money in late 2022 and learned the usefulness of the local stable manifold theorem. During my studies, I noticed that there is often hand-waving in applied works. For instance, Blanchard & Fishcer (1989, p. 268, Endnote 16) state

Care must be taken in using a phase diagram to analyze the dynamics of a difference equation system. […] Thus we must check in this case whether the system is indeed saddle point stable […]. This check is left to the reader.

Furthermore, during the review process of our JPE paper on the Bubble Necessity Theorem, we learned about Scheinkman (1980)’s sufficient condition for local determinacy that involves the curvature of the utility function, which was a bit mysterious. So after resubmitting our paper to JPE at the end of December 2023, we started to work on a complete analysis of the local determinacy of equilibria in Tirole (1985)’s model. It turns out that Scheinkman’s sufficient condition does not generalize to production economies: there are robust examples with arbitrary utility functions in which the nonmonetary steady state is locally determinate or indeterminate. In contrast, the monetary steady state is locally determinate under fairly weak conditions.



Operations Research (Econ 172B)

Undergraduate, UCSD, 2015

This course covers some topics in operations research, such as convex analysis, nonlinear programming, and dynamic programming. I do not currently teach this course.