Wolfgang Karl Schief

Natural classes of integrability-preserving reductions of a 4+4-dimensional generalisation (TED equation) of the general heavenly equation are recorded. In particular, these reductions lead to integrable ‘deformations’ of various other avatars of the heavenly equation governing self-dual Einstein spaces. The known deformed heavenly equations which give rise to half-flat conformal structures are retrieved in this manner. Moreover, Plebański’s link between the first and second heavenly equations is extended to their deformed counterparts.

It is established that the classical Taub system governing isentropic planar motions of perfect fluids in special relativity may be formulated entirely in soliton-theoretic terms. The well-established tools of integrable systems theory may therefore be used to produce large classes of solutions given in parametric form. As an illustration, the general solution of the Taub system associated with a privileged pressure-density constitutive law is generated by applying a binary Darboux transformation to the seed solution of the Taub system descriptive of extreme fluids.

A geometric decomposition previously adopted in areas of nonlinear continuum mechanics and soliton theory is applied to a two-dimensional time-independent magnetohydrodynamic system to obtain reduction to a canonical third order nonlinear equation subject to a constraint. Novel classes of exact solutions to the magnetohydrodynamic system are thereby constructed. Connection is made with kinematic conditions which attend certain steady two-dimensional motions of fibre-reinforced viscous fluids.

An intrinsic geometric decomposition is applied to provide via a canonical third-order nonlinear equation a connection between 2+1-dimensional fibre-reinforced motions of a fluid and a magnetohydrodynamic system. A Lagrange-type parametrisation is introduced, whereby both geometric and algebraic properties of certain non-steady magnetohydrodynamic motions may be established.

It is shown that the kinematic equations governing steady motions of an ideal fibre-reinforced fluid in a curved stratum may be expressed entirely in terms of the intrinsic Gauss equation, which assumes the form of a partial differential equation of third order, for the surface representing the stratum. In particular, the approach adopted here leads to natural non-classical orthogonal coordinate systems on surfaces of constant Gaussian curvature with one family of coordinate lines representing the fibres. Integrable cases are isolated by requiring that the Gauss equation be compatible with another third-order hyperbolic differential equation. In particular, a variant of the integrable Tzitzéica equation is derived which encodes orthogonal coordinate systems on pseudospherical surfaces. This third-order equation is related to the Tzitzéica equation by an analogue of the Miura transformation for the (modified) Korteweg–de Vries equation. Finally, the formalism developed in this paper is illustrated by focussing on the simplest ‘fluid sheets’ of constant Gaussian curvature, namely the plane, sphere and pseudosphere.

Eigenfunctions are shown to constitute privileged coordinates of self-dual Einstein spaces with the underlying governing equation being revealed as the general heavenly equation. The formalism developed here may be used to link algorithmically a variety of known heavenly equations. In particular, the classical connection between Plebański’s first and second heavenly equations is retrieved and interpreted in terms of eigenfunctions. In addition, connections with travelling wave reductions of the recently introduced TED equation which constitutes a 4 + 4-dimensional integrable generalisation of the general heavenly equation are found. These are obtained by means of (partial) Legendre transformations. As a particular application, we prove that a large class of self-dual Einstein spaces governed by a compatible system of dispersionless Hirota equations is genuinely four-dimensional in that the (generic) metrics do not admit any (proper or non-proper) conformal Killing vectors. This generalises the known link between a particular class of self-dual Einstein spaces and the dispersionless Hirota equation encoding three-dimensional Einstein–Weyl geometries.

A classical system of conservation laws descriptive of relativistic gasdynamics is examined. In the two-dimensional stationary case, the system is shown to be invariant under a novel multi-parameter class of reciprocal transformations. The class of invariant transformations originally obtained by Bateman in non-relativistic gasdynamics in connection with lift and drag phenomena is retrieved as a reduction in the classical limit. In the general 3+1-dimensional case, it is demonstrated that Synge’s geometric characterization of the pressure being constant along streamlines encapsulates a three-dimensional extension of an integrable Heisenberg spin equation.

It is shown that the kinematic system describing planar non-steady motions of ideal fibre-reinforced fluids may be reduced to a single two-dimensional third-order partial differential equation in which time enters parametrically. A procedure is established which maps steady motions to non-steady motions. The resulting motions inherit their hidden integrable structure from the steady case. The formalism presented here also readily recovers the connection with the scattering problem of the modified Korteweg–de Vries (mKdV) hierarchy established in previous work.