Default Ambiguity: Finding the Best Solution to the Clearing Problem
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We study financial networks with debt contracts and credit default swaps between specific pairs of banks. Given such a financial system, we want to decide which of the banks are in default, and how much of their liabilities these defaulting banks can pay. There can easily be multiple different solutions to this problem, leading to a situation of default ambiguity and a range of possible solutions to implement for a financial authority. In this paper, we study the general properties of the solution space of such financial systems, and analyze a wide range of reasonable objective functions for selecting from the set of solutions. Examples of such objective functions include minimizing the number of defaulting banks, minimizing the amount of unpaid debt, maximizing the number of satisfied banks, maximizing the equity of a specific bank, finding the most balanced distribution of equity, and many others. We show that for all of these objective functions, it is not only NP-hard to find the optimal solution, but it is also NP-hard to approximate this optimum: for each objective function, we show an inapproximability either to an n^1/2-ϵ or to an n^1/4-ϵ factor for any ϵ>0, with n denoting the number of banks in the system. Thus even if an authority has clear criteria to select a solution in case of default ambiguity, it is computationally intractable to find a solution that is reasonably good in terms of this criteria. We also show that our hardness results hold in a wide range of different model variants.
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