
on Contract Theory and Applications 
By:  Henri Pag\`es; Dylan Possamai 
Abstract:  In this paper, we take up the analysis of a principal/agent model with moral hazard introduced in \cite{pages}, with optimal contracting between competitive investors and an impatient bank monitoring a pool of longterm loans subject to Markovian contagion. We provide here a comprehensive mathematical formulation of the model and show using martingale arguments in the spirit of Sannikov \cite{san} how the maximization problem with implicit constraints faced by investors can be reduced to a classic stochastic control problem. The approach has the advantage of avoiding the more general techniques based on forwardbackward stochastic differential equations described in \cite{cviz} and leads to a simple recursive system of HamiltonJacobiBellman equations. We provide a solution to our problem by a verification argument and give an explicit description of both the value function and the optimal contract. Finally, we study the limit case where the bank is no longer impatient. 
Date:  2012–02 
URL:  http://d.repec.org/n?u=RePEc:arx:papers:1202.2076&r=cta 
By:  Alina Beygelzimer; John Langford; David Pennock 
Abstract:  In evaluating prediction markets (and other crowdprediction mechanisms), investigators have repeatedly observed a socalled "wisdom of crowds" effect, which roughly says that the average of participants performs much better than the average participant. The market pricean average or at least aggregate of traders' beliefsoffers a better estimate than most any individual trader's opinion. In this paper, we ask a stronger question: how does the market price compare to the best trader's belief, not just the average trader. We measure the market's worstcase log regret, a notion common in machine learning theory. To arrive at a meaningful answer, we need to assume something about how traders behave. We suppose that every trader optimizes according to the Kelly criteria, a strategy that provably maximizes the compound growth of wealth over an (infinite) sequence of market interactions. We show several consequences. First, the market prediction is a wealthweighted average of the individual participants' beliefs. Second, the market learns at the optimal rate, the market price reacts exactly as if updating according to Bayes' Law, and the market prediction has low worstcase log regret to the best individual participant. We simulate a sequence of markets where an underlying true probability exists, showing that the market converges to the true objective frequency as if updating a Beta distribution, as the theory predicts. If agents adopt a fractional Kelly criteria, a common practical variant, we show that agents behave like fullKelly agents with beliefs weighted between their own and the market's, and that the market price converges to a timediscounted frequency. Our analysis provides a new justification for fractional Kelly betting, a strategy widely used in practice for adhoc reasons. Finally, we propose a method for an agent to learn her own optimal Kelly fraction. 
Date:  2012–01 
URL:  http://d.repec.org/n?u=RePEc:arx:papers:1201.6655&r=cta 
By:  Masaaki Fujii; Akihiko Takahashi 
Abstract:  In this work, we apply our newly proposed perturbative expansion technique to a quadratic growth FBSDE appearing in an incomplete market with stochastic volatility that is not perfectly hedgeable. By combining standard asymptotic expansion technique for the underlying volatility process, we derive explicit expression for the solution of the FBSDE up to the third order of volatilityofvolatility, which can be directly translated into the optimal investment strategy. We compare our approximation with the exact solution, which is known to be derived by the ColeHopf transformation in this popular setup. The result is very encouraging and shows good accuracy of the approximation up to quite long maturities. Since our new methodology can be extended straightforwardly to multidimensional setups, we expect it will open real possibilities to obtain explicit optimal portfolios or hedging strategies under realistic assumptions. 
Date:  2012–02 
URL:  http://d.repec.org/n?u=RePEc:arx:papers:1202.0608&r=cta 
By:  Peter Kratz; Torsten Sch\"oneborn 
Abstract:  We consider an illiquid financial market where a riskaverse investor has to liquidate a large portfolio within a finite time horizon [0,T] and can trade continuously at a traditional exchange (the "primary venue") and in a dark pool. At the primary venue, trading yields a linear price impact. In the dark pool, no price impact costs arise but order execution is uncertain, modeled by a multidimensional Poisson process. We characterize the costs of trading by a linearquadratic functional which incorporates both the price impact costs of trading at the primary exchange and the market risk of the position. The liquidation constraint implies a singularity of the value function of the resulting minimization problem at the terminal time T. Via the HJB equation and a quadratic ansatz, we obtain a candidate for the value function which is the limit of a sequence of solutions of initial value problems for a matrix differential equation. Although the differential equation is not a Riccati equation, we are able to show that this limit exists by using an appropriate matrix inequality and a comparison result for Riccati equations. Additionally, we obtain upper and lower bounds of the solutions of the initial value problems, which allow us to prove a verification theorem. If a single asset position is to be liquidated, the investor slowly trades out of her position at the primary venue, with the remainder being placed in the dark pool at any point in time. For multiasset liquidations this is generally not optimal, and the optimal strategy depends strongly on the correlation of the assets. 
Date:  2012–01 
URL:  http://d.repec.org/n?u=RePEc:arx:papers:1201.6130&r=cta 