Divergence of combinatorial averages and the unboundedness of the trilinear Hilbert transform

摘要

We consider multilinear averages in ergodic theory and harmonic analysis and prove their divergence in some range of L-p spaces. This contrasts with the positive behavior exhibited by these averages in a different range, as proved in Demeter et al [Maximal multilinear operators. Trans. Amer. Math. Soc. 360(9) (2008), 4989-5042]. We also prove that the trilinear Hilbert transform is unbounded in a similar range of LP spaces. The principle underlying these constructions is stated, setting the stage for more general results.