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3. I. M. Gelfand and D. B. Fuks, Cohomologies of the Lie algebra of vector fields on the circle, Funct. Anal. Appl. 2(1968), no. 4, 92–93; English translation 342-343.
4. V. G. Kac, Some problems on infinite dimensional Lie algebras and their representations, pp. 117-126 in Lie Algebras and Related Topics, Springer Lecture Notes no. 933, 1982.
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