### Abstract

We study the equation $\mathcal{E}_{\mathrm{fc}}$ of flat connections in a given fiber bundle and discover a specific geometric structure on it, which we call a \emph{flat representation}. We generalize this notion to arbitrary PDE and prove that flat representations of an equation $\mathcal{E}$ are in $1$-$1$ correspondence with morphisms $\varphi\colon\mathcal{E}\to\mathcal{E}_{\mathrm{fc}}$, where $\mathcal{E}$ and $\mathcal{E}_{\mathrm{fc}}$ are treated as submanifolds of infinite jet spaces. We show that flat representations include several known types of zero-\hspace{0pt} curvature formulations of PDEs. In particular, the Lax pairs of the self-dual Yang--Mills equations and their reductions are of this type. With each flat representation $\varphi$ we associate a complex $C_\varphi$ of vector-\hspace{0pt} valued differential forms such that $H^1(C_\varphi)$ describes infinitesimal deformations of the flat structure, which are responsible, in particular, for parameters in B\"{a}cklund transformations. In addition, each higher infinitesimal symmetry $S$ of $\mathcal{E}$ defines a $1$-\hspace{0pt} cocycle $c_S$ of $C_\varphi$. Symmetries with exact $c_S$ form a subalgebra reflecting some geometric properties of $\mathcal{E}$ and $\varphi$. We show that the complex corresponding to $\mathcal{E}_{\mathrm{fc}}$ itself is $0$-\hspace{0pt} acyclic and $1$-\hspace{0pt} acyclic (independently of the bundle topology), which means that higher symmetries of $\mathcal{E}_{\mathrm{fc}}$ are exhausted by generalized gauge ones, and compute the bracket on $0$-\hspace{0pt} cochains induced by commutation of symmetries.

Original language | Undefined |
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Place of Publication | Enschede |

Publisher | Fundamentele Analyse |

Number of pages | 33 |

Publication status | Published - 2002 |

### Publication series

Name | memorandum |
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Publisher | Department of Applied Mathematics, University of Twente |

No. | 1639 |

ISSN (Print) | 0169-2690 |

### Keywords

- MSC-37K25
- MSC-37K35
- MSC-58J10
- MSC-53C05
- EWI-3459
- IR-65826
- METIS-214163

## Cite this

Igonine, S., Kersten, P. H. M., & Krasil'shchik, I. (2002).

*On symmetries and cohomological invariants of equations possessing flat representations*. (memorandum; No. 1639). Enschede: Fundamentele Analyse.