Webs, Nijenhuis operators, and heavenly PDEs

Abstract: In 1989 Mason and Newman proved that there is a 1-1-correspondence between self-dual metrics satisfying Einstein vacuum equation (in complex case or in neutral signature) and pairs of commuting parameter depending vector fields $X_1(\lambda),X_2(\lambda)$ which are divergence free with respect to some volume form. Earlier (in 1975) Pleba\'nski showed instances of such vector fields depending of one function of four variables satisfying the so-called I or II Pleba\'nski heavenly PDEs. Other PDEs leading to Mason--Newman vector fields are also known in the literature: Husain--Park (1992--94), Schief (1996). In this paper we discuss these matters in the context of the web theory, i.e. theory of collections of foliations on a manifold, understood from the point of view of Nijenhuis operators. In particular we show how to apply this theory for constructing new heavenly'' PDEs based on different normal forms of Nijenhuis operators in 4D, which are integrable similarly to their predecessors. Relation with the Hirota dispersionless systems of PDEs and the corresponding Veronese webs, which was recently observed by Konopelchenko--Schief--Szereszewski, is established in all the cases. We also discuss some higher dimensional generalizations of theheavelny'' PDEs and the existence of related vacuum Einstein metrics in 4D-case.