12 research outputs found
Recursion Operators and Tri-Hamiltonian Structure of the First Heavenly Equation of Pleba\'nski
We present first heavenly equation of Pleba\'nski in a two-component
evolutionary form and obtain Lagrangian and Hamiltonian representations of this
system. We study all point symmetries of the two-component system and, using
the inverse Noether theorem in the Hamiltonian form, obtain all the integrals
of motion corresponding to each variational (Noether) symmetry. We derive two
linearly independent recursion operators for symmetries of this system related
by a discrete symmetry of both the two-component system and its symmetry
condition. Acting by these operators on the first Hamiltonian operator we
obtain second and third Hamiltonian operators. However, we were not able to
find Hamiltonian densities corresponding to the latter two operators.
Therefore, we construct two recursion operators, which are either even or odd,
respectively, under the above-mentioned discrete symmetry. Acting with them on
, we generate another two Hamiltonian operators and and find
the corresponding Hamiltonian densities, thus obtaining second and third
Hamiltonian representations for the first heavenly equation in a two-component
form. Using P. Olver's theory of the functional multi-vectors, we check that
the linear combination of , and with arbitrary constant
coefficients satisfies Jacobi identities. Since their skew symmetry is obvious,
these three operators are compatible Hamiltonian operators and hence we obtain
a tri-Hamiltonian representation of the first heavenly equation. Our
well-founded conjecture applied here is that P. Olver's method works fine for
nonlocal operators and our proof of the Jacobi identities and bi-Hamiltonian
structures crucially depends on the validity of this conjecture.Comment: Some text overlap with our paper arXiv:1510.03666 is caused by our
use here of basically the same method for discovering the Hamiltonian and
bi-Hamiltonian structures of the equation, but the equation considered here
and the results are totally different from arXiv:1510.0366
Recursions of Symmetry Orbits and Reduction without Reduction
We consider a four-dimensional PDE possessing partner symmetries mainly on
the example of complex Monge-Amp\`ere equation (CMA). We use simultaneously two
pairs of symmetries related by a recursion relation, which are mutually complex
conjugate for CMA. For both pairs of partner symmetries, using Lie equations,
we introduce explicitly group parameters as additional variables, replacing
symmetry characteristics and their complex conjugates by derivatives of the
unknown with respect to group parameters. We study the resulting system of six
equations in the eight-dimensional space, that includes CMA, four equations of
the recursion between partner symmetries and one integrability condition of
this system. We use point symmetries of this extended system for performing its
symmetry reduction with respect to group parameters that facilitates solving
the extended system. This procedure does not imply a reduction in the number of
physical variables and hence we end up with orbits of non-invariant solutions
of CMA, generated by one partner symmetry, not used in the reduction. These
solutions are determined by six linear equations with constant coefficients in
the five-dimensional space which are obtained by a three-dimensional Legendre
transformation of the reduced extended system. We present algebraic and
exponential examples of such solutions that govern Legendre-transformed
Ricci-flat K\"ahler metrics with no Killing vectors. A similar procedure is
briefly outlined for Husain equation
Symmetry analysis and exact solutions of modified Brans-Dicke cosmological equations
We perform a symmetry analysis of modified Brans-Dicke cosmological equations
and present exact solutions. We discuss how the solutions may help to build
models of cosmology where, for the early universe, the expansion is linear and
the equation of state just changes the expansion velocity but not the
linearity. For the late universe the expansion is exponential and the effect of
the equation of state on the rate of expansion is just to change the constant
Hubble parameter.Comment: LaTeX2e source file, 14 pages, 7 reference