Poincaré–Birkhoff–Witt's theorem for Leibniz algebras. The dialgebras have been recently introduced by J. L. Loday. For any Leibniz algebra is defined an enveloping dialgebra which has a good universal property. In this work we give, in the category of dialgebras, a Poincaré–Birkhoff–Witt's theorem for Leibniz algebras: Let g be a Leibniz algebra, g Lie the associated universal Lie algebra and S(g Lie ) the symmetric algebra on the K-module g Lie . Let grUd(g) be the associated graded dialgebra of the enveloping dialgebra Ud(g) of g. Then, if g is free as K-module, there is an isomorphism of graded dialgebras S(g Lie ) ⊗ g = grUd(g).
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