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Partially supported by NSF grant No. MPS-01892-01
The authors were supported by NSF grants at the Institute for Advanced Study
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Menikoff, A., Sjöstrand, J. On the eigenvalues of a class of hypoelliptic operators. Math. Ann. 235, 55–85 (1978). https://doi.org/10.1007/BF01421593
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DOI: https://doi.org/10.1007/BF01421593