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Transactions of the American Mathematical Society
Transactions of the American Mathematical Society
ISSN 1088-6850(online) ISSN 0002-9947(print)

 

Vessiot structure for manifolds of $(p,q)$-hyperbolic type: Darboux integrability and symmetry


Author: Peter J. Vassiliou
Journal: Trans. Amer. Math. Soc. 353 (2001), 1705-1739
MSC (2000): Primary 58D27, 58J45, 58J70; Secondary 35L70
Published electronically: December 18, 2000
MathSciNet review: 1813593
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Abstract:

It is well known that if a scalar second order hyperbolic partial differential equation in two independent variables is Darboux integrable, then its local Cauchy problem may be solved by ordinary differential equations. In addition, such an equation has infinitely many non-trivial conservation laws. Moreover, Darboux integrable equations have properties in common with infinite dimensional completely integrable systems.

In this paper we employ a geometric object intrinsically associated with any hyperbolic partial differential equation, its hyperbolic structure, to study the Darboux integrability of the class $E$ of semilinear second order hyperbolic partial differential equations in one dependent and two independent variables. It is shown that the problem of classifying the Darboux integrable equations in $E$ contains, as a subproblem, that of classifying the manifolds of $(p,q)$-hyperbolic type of rank 4 and dimension $2k+3$, $k\ge2$; $p=2,q\ge 2$.

In turn, it is shown that the problem of classifying these manifolds in the two (lowest) cases $(p,q)=(2,2),(2,3)$ contains, as a subproblem, the classification problem for Lie groups. This generalizes classical results of E. Vessiot.

The main result is that if an equation in $E$ is (2,2)- or (2,3)-Darboux integrable on the $k$-jets, $k\ge 2$, then its intrinsic hyperbolic structure admits a Lie group of symmetries of dimension $2k-1$ or $2k-2$, respectively. It follows that part of the moduli space for the Darboux integrable equations in $E$ is determined by isomorphism classes of Lie groups.

The Lie group in question is the group of automorphisms of the characteristic systems of the given equation which leaves invariant the foliation induced by the characteristic (or, Riemann) invariants of the equation, the tangential characteristic symmetries. The isomorphism class of the tangential characteristic symmetries is a contact invariant of the corresponding Darboux integrable partial differential equation.


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Additional Information

Peter J. Vassiliou
Affiliation: School of Mathematics and Statistics, University of Canberra, Canberra ACT, Australia, 2601
Email: pierre@ise.canberra.edu.au

DOI: http://dx.doi.org/10.1090/S0002-9947-00-02670-2
PII: S 0002-9947(00)02670-2
Received by editor(s): November 17, 1998
Received by editor(s) in revised form: December 16, 1999
Published electronically: December 18, 2000
Article copyright: © Copyright 2000 American Mathematical Society