We study the complexity of the classic Hylland-Zeckhauser scheme [HZ79] for one-sided matching markets. We show that the problem of finding an $epsilon$-approximate equilibrium in the HZ scheme is PPAD-hard, and this holds even when $epsilon$ is polynomially small and when each agent has no more than four distinct utility values. Our hardness result, when combined with the PPAD membership result of [VY21], resolves the approximation complexity of the HZ scheme. We also show that the problem of approximating the optimal social welfare (the weight of the matching) achievable by HZ equilibria within a certain constant factor is NP-hard.
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