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Supported in part by NSF and the John Simon Guggenheim Foundation.
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Dai, Z.D., Lam, T.Y. Levels in algebra and topology. Commentarii Mathematici Helvetici 59, 376–424 (1984). https://doi.org/10.1007/BF02566358
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DOI: https://doi.org/10.1007/BF02566358