# Research

**Working Papers**

Existence and Uniqueness of Recursive Equilibria with Aggregate and Idiosyncratic Risk

R&R Review of Economic Studies

(paper on SSRN, online appendix, slides)

*In this paper, I study the existence and uniqueness of recursive equilibria in economies with aggregate and idiosyncratic risk. Rather than relying on compactness to establish existence, I exploit the monotonicity property of the equilibrium model and rely on arguments from convex analysis. This methodology does not only give rise to a convergent iterative procedure, but more strikingly, it also yields uniqueness. To illustrate my theoretical results, I establish sufficient conditions for the existence and uniqueness of solutions to the stochastic growth model as in Krusell and Smith (1998) and the heterogeneous-agent exchange economy as in Huggett (1993) with aggregate risk.*

Approximating Equilibria with Ex-Post Heterogeneity and Aggregate Risk

(paper on SSRN, online appendix, code and other supplementary material)

*Dynamic stochastic general equilibrium models with ex-post heterogeneity due to idiosyncratic risk have to be solved numerically. This is a nontrivial task as the cross-sectional distribution of endogenous variables becomes an element of the state space due to aggregate risk. Existing global solution methods often assume bounded rationality in terms of a parametric law of motion of aggregate variables in order to reduce dimensionality. I do not make this assumption and tackle dimensionality by polynomial chaos expansions, a projection technique for square-integrable random variables. This approach results in a nonparametric law of motion of aggregate variables. Moreover, I establish convergence of the proposed algorithm to the rational expectations equilibrium. Economically, I find that higher levels of idiosyncratic risk sharing lead to higher systemic risk, i.e., higher volatility within the ergodic state distribution, and second, heterogeneity leads to an amplification of aggregate risk for sufficiently high levels of risk sharing.*