References
Hörmander, L.: Differential operators of principal type. Math. Ann.140, 124–146 (1960).
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Hörmander, L. Differential equations without solutions. Math. Ann. 140, 169–173 (1960). https://doi.org/10.1007/BF01361142
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DOI: https://doi.org/10.1007/BF01361142