International Mathematics Research Notices Advance Access published online on September 3, 2009
International Mathematics Research Notices, doi:10.1093/imrn/rnp134
© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Integrable Equations of the Dispersionless Hirota type and Hypersurfaces in the Lagrangian Grassmannian
Evgeny Vladimirovich Ferapontov,
Lenos Hadjikos and
Karima Robertovna Khusnutdinova
Department of Mathematical Sciences, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
Correspondence: Correspondence to be sent to: e.v.ferapontov{at}lboro.ac.uk
We investigate integrable second-order equations of the form
which typically arise as the Hirota-type relations
for various (2 + 1)-dimensional dispersionless hierarchies.
Familiar examples include the Boyer–Finley equation

, the potential form of the dispersionless Kadomtsev–Petviashvili
(dKP) equation

, the
dispersionless Hirota equation

, etc. The integrability is understood as the existence of
an infinity of hydrodynamic reductions. We demonstrate that
the natural equivalence group of the problem is isomorphic to
Sp(6), revealing a remarkable correspondence between differential
equations of the above type and hypersurfaces of the Lagrangian
Grassmannian. We prove that the moduli space of integrable equations
of the dispersionless Hirota type is 21-dimensional, and the
action of the equivalence group
Sp(6) on the moduli space has
an open orbit.
Communicated by Prof. Igor Krichever

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