# On the Computational Complexity of Finding a Sparse Wasserstein Barycenter

The discrete Wasserstein barycenter problem is a minimum-cost mass transport problem for a set of probability measures with finite support. In this paper, we show that finding a barycenter of sparse support is hard, even in dimension 2 and for only 3 measures. We prove this claim by showing that a special case of an intimately related decision problem SCMP - does there exist a measure with a non-mass-splitting transport cost and support size below prescribed bounds? - is NP-hard for all rational data. Our proof is based on a reduction from planar 3-dimensional matching and follows a strategy laid out by Spieksma and Woeginger for a reduction to planar, minimum circumference 3-dimensional matching. While we closely mirror the actual steps of their proof, the arguments themselves differ fundamentally due to the complex nature of the discrete barycenter problem. Second, we prove that given a rational measure, an optimal transport to a set of measures can be computed in polynomial time. However, the complexity of this task depends on the size of an encoding of this measure, not only on the input size to the underlying barycenter problem. We show that this distinction causes a simple proof that SCMP is in NP to fail; whether SCMP is in NP remains open.

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