18 research outputs found
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
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
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