The trilinear Hilbert transform is a singular integral that multiplies three inputs translated at distinct fixed rates before integration
against .
For the slopes 1, 2, and 3, it is
Durcik, P. and Thiele, C. "Singular Brascamp-Lieb: A Survey." In Geometric Aspects of Harmonic Analysis (Ed. P. Ciatti
and A. Martini). Cham, Switzerland: Springer, pp. 321-349, 2021. https://doi.org/10.1007/978-3-030-72058-2_9.