Geodesic
Non-Point Invertible Transformations and Integrability of Partial Difference Equations
Symmetry, integrability and geometry: methods and applications, Tome 10 (2014) Cet article a éte moissonné depuis la source Math-Net.Ru

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This article is devoted to the partial difference quad-graph equations that can be represented in the form $\varphi (u(i+1,j),u(i+1,j+1))=\psi (u(i,j),u(i,j+1))$, where the map $(w,z) \rightarrow (\varphi(w,z),\psi(w,z))$ is injective. The transformation $v(i,j)=\varphi (u(i,j),u(i,j+1))$ relates any of such equations to a quad-graph equation. It is proved that this transformation maps Darboux integrable equations of the above form into Darboux integrable equations again and decreases the orders of the transformed integrals by one in the $j$-direction. As an application of this fact, the Darboux integrable equations possessing integrals of the second order in the $j$-direction are described under an additional assumption. The transformation also maps symmetries of the original equations into symmetries of the transformed equations (i.e.preserves the integrability in the sense of the symmetry approach) and acts as a difference substitution for symmetries of a special form. The latter fact allows us to derive necessary conditions of Darboux integrability for the equations defined in the first sentence of the abstract.
Keywords: quad-graph equation; non-point transformation; Darboux integrability; higher symmetry; difference substitution; discrete Liouville equation. 
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     author = {Sergey Ya. Startsev},
     title = {Non-Point {Invertible} {Transformations} and {Integrability} of {Partial} {Difference} {Equations}},
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     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SIGMA_2014_10_a65/}
}
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Sergey Ya. Startsev. Non-Point Invertible Transformations and Integrability of Partial Difference Equations. Symmetry, integrability and geometry: methods and applications, Tome 10 (2014). http://geodesic.mathdoc.fr/item/SIGMA_2014_10_a65/

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