## About

44

Publications

2,600

Reads

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368

Citations

Citations since 2017

Introduction

I currently work at the Department of Applied Mathematics (College of Arts and Sciences), University of Colorado Boulde. My research is focused on Mathematical Finance, Financial Economics, and Applied Probability.

Additional affiliations

September 2013 - August 2016

Education

September 2008 - May 2013

September 2002 - June 2007

September 2002 - June 2007

## Publications

Publications (44)

The paper presents a dynamic theory for time-inconsistent problems of optimal
stopping. The theory is developed under the paradigm of expected discounted
payoff, where the process to stop is continuous and Markovian. We introduce
equilibrium stopping policies, which are imple-mentable stopping rules that
take into account the change of preferences...

In a discrete-time market, we study model-independent superhedging, while the semi-static superhedging portfolio consists of three parts: static positions in liquidly traded vanilla calls, static positions in other tradable, yet possibly less liquid, exotic options, and a dynamic trading strategy in risky assets under certain constraints. By consid...

We consider as given a discrete time financial market with a risky asset and
options written on that asset and determine both the sub- and super-hedging
prices of an American option in the model independent framework of
ArXiv:1305.6008. We obtain the duality of results for the super- and
sub-hedging prices. Then assuming that the path space is comp...

This paper resolves a question proposed in Kardaras and Robertson [Ann. Appl.
Probab. 22 (2012) 1576-1610]: how to invest in a robust growth-optimal way in a
market where precise knowledge of the covariance structure of the underlying
assets is unavailable. Among an appropriate class of admissible covariance
structures, we characterize the optimal...

We consider a zero-sum stochastic differential controller-and-stopper game in
which the state process is a controlled diffusion evolving in a
multi-dimensional Euclidean space. In this game, the controller affects both
the drift and the volatility terms of the state process. Under appropriate
conditions, we show that the game has a value and the va...

For a general entropy-regularized stochastic control problem on an infinite horizon, we prove that the policy improvement algorithm (PIA) converges to an optimal relaxed control. Contrary to the case of a standard stochastic control problem, classical H\"{o}lder estimates cannot ensure the convergence of the PIA, due to the added entropy-regularizi...

Federal student loans are fixed-rate debt contracts with three main special features: (i) borrowers can use income-driven schemes to make payments proportional to their income above subsistence, (ii) after several years of good standing, the remaining balance is forgiven but taxed as ordinary income, and (iii) accrued interest is simple, i.e., not...

This paper approaches the unsupervised learning problem by gradient descent in the space of probability density functions. Our main result shows that along the gradient flow induced by a distribution-dependent ordinary differential equation (ODE), the unknown data distribution emerges as the long-time limit of this flow of densities. That is, one c...

This paper studies a nonzero-sum Dynkin game in discrete time under non-exponential discounting. For both players, there are two intertwined levels of game-theoretic reasoning. First, each player looks for an intra-personal equilibrium among her current and future selves, so as to resolve time inconsistency triggered by non-exponential discounting....

The Byzantine distributed quickest change detection (BDQCD) is studied, where a fusion center monitors the occurrence of an abrupt event through a bunch of distributed sensors that may be compromised. We first consider the binary hypothesis case where there is only one post-change hypothesis and prove a novel converse to the first-order asymptotic...

An unconventional approach for optimal stopping under model ambiguity is introduced. Besides ambiguity itself, we take into account how ambiguity‐averse an agent is. This inclusion of ambiguity attitude, via an ‐maxmin nonlinear expectation, renders the stopping problem time‐inconsistent. We look for subgame perfect equilibrium stopping policies, f...

This paper studies a nonzero-sum Dynkin game in discrete time under non-exponential discounting. For both players, there are two levels of game-theoretic reasoning intertwined. First, each player looks for an intra-personal equilibrium among her current and future selves, so as to resolve time inconsistency triggered by non-exponential discounting....

A new definition of continuous-time equilibrium controls is introduced. As opposed to the standard definition, which involves a derivative-type operation, the new definition parallels how a discrete-time equilibrium is defined and allows for unambiguous economic interpretation. The terms “strong equilibria” and “weak equilibria” are coined for cont...

We study an optimal stopping problem under non-exponential discounting, where the state process is a multi-dimensional continuous strong Markov process. The discount function is taken to be log sub-additive, capturing decreasing impatience in behavioral economics. On strength of probabilistic potential theory, we establish the existence of an optim...

For an infinite‐horizon continuous‐time optimal stopping problem under nonexponential discounting, we look for an optimal equilibrium, which generates larger values than any other equilibrium does on the entire state space. When the discount function is log sub‐additive and the state process is one‐dimensional, an optimal equilibrium is constructed...

This paper investigates optimal consumption, investment, and healthcare spending under Epstein-Zin preferences. Given consumption and healthcare spending plans, Epstein-Zin utilities are defined over an agent's random lifetime, partially controllable by the agent as healthcare reduces Gompertz' natural growth rate of mortality. In a Black-Scholes m...

In a discrete-time financial market, a generalized duality is established for model-free superhedging, given marginal distributions of the underlying asset. Contrary to prior studies, we do not require contingent claims to be upper semicontinuous, allowing for upper semi-analytic ones. The generalized duality stipulates an extended version of risk-...

The Byzantine distributed quickest change detection (BDQCD) is studied, where a fusion center monitors the occurrence of an abrupt event through a bunch of distributed sensors that may be compromised. We first consider the binary hypothesis case where there is only one post-change hypothesis and prove a novel converse to the first-order asymptotic...

We consider the problem of stopping a diffusion process with a payoff functional that renders the problem time‐inconsistent. We study stopping decisions of naïve agents who reoptimize continuously in time, as well as equilibrium strategies of sophisticated agents who anticipate but lack control over their future selves' behaviors. When the state pr...

An unconventional approach for optimal stopping under model ambiguity is introduced. Besides ambiguity itself, we take into account how ambiguity-averse an agent is. This inclusion of ambiguity attitude, via an $\alpha$-maxmin nonlinear expectation, renders the stopping problem time-inconsistent. We look for subgame perfect equilibrium stopping pol...

This paper solves the problem of optimal dynamic investment, consumption and healthcare spending with isoelastic utility, when natural mortality grows exponentially to reflect the Gompertz law and investment opportunities are constant. Healthcare slows the natural growth of mortality, indirectly increasing utility from consumption through longer li...

This paper solves the consumption-investment problem with Epstein-Zin utility on a random horizon. In an incomplete market, we take the random horizon to be a stopping time adapted to the market filtration, generated by all observable, but not necessarily tradable, state processes. Contrary to prior studies, we do not impose any fixed upper bound f...

This paper solves the problem of optimal dynamic consumption, investment, and healthcare spending with isoelastic utility, when natural mortality grows exponentially to reflect Gompertz' law and investment opportunities are constant. Healthcare slows the natural growth of mortality, indirectly increasing utility from consumption through longer life...

This paper investigates optimal consumption in the stochastic Ramsey problem with the Cobb-Douglas production function. Contrary to prior studies, we allow for general consumption processes, without any a priori boundedness constraint. A nonstandard stochastic diferential equation, with neither Lipschitz continuity nor linear growth, specifes the d...

A new definition of continuous-time equilibrium controls is introduced. As opposed to the standard definition, which involves a derivative-type operation, the new definition parallels how a discrete-time equilibrium is defined, and allows for unambiguous economic interpretation. The terms "strong equilibria" and "weak equilibria" are coined for con...

This paper investigates optimal consumption in the stochastic Ramsey problem with the Cobb-Douglas production function. Contrary to previous studies, we allow for general consumption processes, without any a priori boundedness constraint. The associated value function is characterized as the unique classical solution to a nonlinear elliptic equatio...

Under non-exponential discounting, we develop a dynamic theory for stopping problems in continuous time. Our framework covers discount functions that induce decreasing impatience. Due to the inherent time inconsistency, we look for equilibrium stopping policies, formulated as fixed points of an operator. Under appropriate conditions, fixed-point it...

For an infinite-horizon continuous-time optimal stopping problem under non-exponential discounting, we look for an optimal equilibrium, which generates larger values than any other equilibrium does on the entire state space. When the discount function is log sub-additive and the state process is one-dimensional, an optimal equilibrium is constructe...

We consider the problem of stopping a diffusion process with a payoff functional involving probability distortion. The problem is inherently time-inconsistent as the level of distortion of a same event changes over time. We study stopping decisions of naive agents who reoptimize continuously in time, as well as equilibrium strategies of sophisticat...

We study an infinite-horizon discrete-time optimal stopping problem under non-exponential discounting. A new method, which we call the iterative approach, is developed to find subgame perfect Nash equilibriums. When the discount function induces decreasing impatience, we establish the existence of an equilibrium through fixed-point iterations. More...

We study the stochastic solution to a Cauchy problem for a degenerate parabolic equation arising from option pricing. When the diffusion coefficient of the underlying price process is locally Hölder continuous with exponent , the stochastic solution, which represents the price of a European option, is shown to be a classical solution to the Cauchy...

Health-care slows the natural growth of mortality, indirectly increasing utility from consumption through longer lifetimes. This paper solves the problem of optimal dynamic consumption and healthcare spending with isoelastic utility, when natural mortality grows exponentially to reflect the Gompertz' law. Optimal consumption and healthcare imply an...

In a discrete-time market, we study model-independent superhedging where the semi-static superhedging portfolio consists of three parts: static positions in liquidly traded vanilla calls, static positions in other tradable, yet possibly less liquid, exotic options, and a dynamic trading strategy in risky assets under certain constraints. By conside...

We consider the stochastic solution to a Cauchy problem corresponding to a
nonnegative diffusion with zero drift, which represents a price process under
some risk-neutral measure. When the diffusion coefficient is locally H\"{o}lder
continuous with some exponent in (0,1], the stochastic solution is shown to be
a classical solution. A comparison the...

This thesis is devoted to PDE characterization for stochastic control problems when the classical methodology of dynamic programming does not work. Under the framework of viscosity solutions, a dynamic programming principle (DPP) serves as the tool to associate a (nonlinear) PDE to a stochastic control problem. Unfortunately, a DPP is in general di...

Our goal is to resolve a problem proposed by Fernholz and Karatzas [On
optimal arbitrage (2008) Columbia Univ.]: to characterize the minimum amount of
initial capital with which an investor can beat the market portfolio with a
certain probability, as a function of the market configuration and time to
maturity. We show that this value function is th...

## Projects

Projects (3)

Optimal stopping against an adversarial Nature, Skorohod embedding, regularity results for free boundary problems, quickest detection, devising and convergence of numerical schemes.

Pricing and hedging under model uncertainty, stochastic optimal control and stopping with model uncertainty