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- [8] HYPOELLIPTICITY AND NONHYPOELLIPTICITY FOR SUMS OF SQUARES OF COMPLEX VECTOR FIELDS ANALYSIS & PDE, 2013, 6 (02): : 371 - 445
- [9] SUMS OF SQUARES FORMULAS WITH INTEGER COEFFICIENTS CANADIAN MATHEMATICAL BULLETIN-BULLETIN CANADIEN DE MATHEMATIQUES, 1987, 30 (03): : 318 - 324