34 research outputs found
On integrability in Grassmann geometries: integrable systems associated with fourfolds in Gr(3, 5)
Let Gr(d; n) be the Grassmannian of d-dimensional linear subspaces of an n-dimensional vector space V n. A submanifold X Gr(d; n) gives rise to a differential system ⊂(X)
that governs d-dimensional submanifolds of V n whose Gaussian image is contained in X.
Systems of the form Σ(X) appear in numerous applications in continuum mechanics, theory
of integrable systems, general relativity and differential geometry. They include such wellknown
examples as the dispersionless Kadomtsev-Petviashvili equation, the Boyer-Finley
equation, Plebansky's heavenly equations, and so on. In this paper we concentrate on the particularly interesting case of this construction
where X is a fourfold in Gr(3; 5). Our main goal is to investigate differential-geometric and
integrability aspects of the corresponding systems Σ(X). We demonstrate the equivalence
of several approaches to dispersionless integrability such as • the method of hydrodynamic reductions, • the method of dispersionless Lax pairs, • integrability on solutions, based on the requirement that the characteristic variety of system Σ(X) defines an Einstein-Weyl geometry on every solution,
• integrability on equation, meaning integrability (in twistor-theoretic sense) of the canonical
GL(2;R) structure induced on a fourfold X ⊂ Gr(3; 5). All these seemingly different approaches lead to one and the same class of integrable systems
Σ(X). We prove that the moduli space of such systems is 6-dimensional. We give a complete
description of linearisable systems (the corresponding fourfold X is a linear section of
Gr(3; 5)) and linearly degenerate systems (the corresponding fourfold X is the image of a
quadratic map P4 99K Gr(3; 5)). The fourfolds corresponding to `generic' integrable systems
are not algebraic, and can be parametrised by generalised hypergeometric functions
Free CR distributions
There are only some exceptional CR dimensions and codimensions such that the
geometries enjoy a discrete classification of the pointwise types of the
homogeneous models. The cases of CR dimensions and codimensions are
among the very few possibilities of the so called parabolic geometries. Indeed,
the homogeneous model turns out to be \PSU(n+1,n)/P with a suitable parabolic
subgroup . We study the geometric properties of such real
-dimensional submanifolds in for all . In
particular we show that the fundamental invariant is of torsion type, we
provide its explicit computation, and we discuss an analogy to the Fefferman
construction of a circle bundle in the hypersurface type CR geometry
On the integrability of symplectic Monge-Amp\'ere equations
Let u be a function of n independent variables x^1, ..., x^n, and U=(u_{ij})
the Hessian matrix of u. The symplectic Monge-Ampere equation is defined as a
linear relation among all possible minors of U. Particular examples include the
equation det U=1 governing improper affine spheres and the so-called heavenly
equation, u_{13}u_{24}-u_{23}u_{14}=1, describing self-dual Ricci-flat
4-manifolds. In this paper we classify integrable symplectic Monge-Ampere
equations in four dimensions (for n=3 the integrability of such equations is
known to be equivalent to their linearisability). This problem can be
reformulated geometrically as the classification of 'maximally singular'
hyperplane sections of the Plucker embedding of the Lagrangian Grassmannian. We
formulate a conjecture that any integrable equation of the form F(u_{ij})=0 in
more than three dimensions is necessarily of the symplectic Monge-Ampere type.Comment: 20 pages; added more details of proof
Ordinary differential equations which linearize on differentiation
In this short note we discuss ordinary differential equations which linearize
upon one (or more) differentiations. Although the subject is fairly elementary,
equations of this type arise naturally in the context of integrable systems.Comment: 9 page
An algebraic method of classification of S-integrable discrete models
A method of classification of integrable equations on quad-graphs is
discussed based on algebraic ideas. We assign a Lie ring to the equation and
study the function describing the dimensions of linear spaces spanned by
multiple commutators of the ring generators. For the generic case this function
grows exponentially. Examples show that for integrable equations it grows
slower. We propose a classification scheme based on this observation.Comment: 11 pages, workshop "Nonlinear Physics. Theory and Experiment VI",
submitted to TM
Twistor geometry of a pair of second order ODEs
We discuss the twistor correspondence between path geometries in three
dimensions with vanishing Wilczynski invariants and anti-self-dual conformal
structures of signature . We show how to reconstruct a system of ODEs
with vanishing invariants for a given conformal structure, highlighting the
Ricci-flat case in particular. Using this framework, we give a new derivation
of the Wilczynski invariants for a system of ODEs whose solution space is
endowed with a conformal structure. We explain how to reconstruct the conformal
structure directly from the integral curves, and present new examples of
systems of ODEs with point symmetry algebra of dimension four and greater which
give rise to anti--self--dual structures with conformal symmetry algebra of the
same dimension. Some of these examples are analogues of plane wave
space--times in General Relativity. Finally we discuss a variational principle
for twistor curves arising from the Finsler structures with scalar flag
curvature.Comment: Final version to appear in the Communications in Mathematical
Physics. The procedure of recovering a system of torsion-fee ODEs from the
heavenly equation has been clarified. The proof of Prop 7.1 has been
expanded. Dedicated to Mike Eastwood on the occasion of his 60th birthda
Classification of integrable discrete Klein-Gordon models
The Lie algebraic integrability test is applied to the problem of
classification of integrable Klein-Gordon type equations on quad-graphs. The
list of equations passing the test is presented containing several well-known
integrable models. A new integrable example is found, its higher symmetry is
presented.Comment: 12 pages, submitted to Physica Script
Explicit differential characterization of the Newtonian free particle system in m > 1 dependent variables
In 1883, as an early result, Sophus Lie established an explicit necessary and
sufficient condition for an analytic second order ordinary differential
equation y_xx = F(x,y,y_x) to be equivalent, through a point transformation
(x,y) --> (X(x,y), Y(x,y)), to the Newtonian free particle equation Y_XX = 0.
This result, preliminary to the deep group-theoretic classification of second
order analytic ordinary differential equations, was parachieved later in 1896
by Arthur Tresse, a French student of S. Lie. In the present paper, following
closely the original strategy of proof of S. Lie, which we firstly expose and
restitute in length, we generalize this explicit characterization to the case
of several second order ordinary differential equations. Let K=R or C, or more
generally any field of characteristic zero equipped with a valuation, so that
K-analytic functions make sense. Let x in K, let m > 1, let y := (y^1, ...,
y^m) in K^m and let y_xx^j = F^j(x,y,y_x^l), j = 1,...,m be a collection of m
analytic second order ordinary differential equations, in general nonlinear. We
provide an explicit necessary and sufficient condition in order that this
system is equivalent, under a point transformation (x, y^1, ..., y^m) -->
(X(x,y), Y^1(x,y),..., Y^m(x, y)), to the Newtonian free particle system Y_XX^1
= ... = Y_XX^m = 0. Strikingly, the (complicated) differential system that we
obtain is of first order in the case m > 1, whereas it is of second order in S.
Lie's original case m = 1.Comment: 76 pages, no figur
Fundamental invariants of systems of ODEs of higher order
We find the complete set of fundamental invariants for systems of ordinary differential equations of order >= 4 under the group of point transformations generalizing similar results for contact invariants of a single ODE and point invariants of systems of the second and the third order. It turns out that starting from systems of order (k + 1) >= 4, the complete set of fundamental invariants is formed by k generalized Wilczynski invariants coming from the linearized system and an additional invariant of degree 2