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Galois theory of q-difference in the roots of unity

Hardouin, C (Heidelberg)
Friday 15 May 2009, 09:30-10:30



For $q \in \mathbb{C}*$ non equal to $1$, we denote by $\sigma_q$ the automorphism of $\mathbb{C}(z)$ given by $\sigma_q(f)(z)=f(qz)$. As q goes to $1$, a q-difference equation w.r.t. $\sigma_q$ goes to a differential equation. The theory related to this fact is also called \textit{confluence} and one part of its study is the behaviour of the related Galois groups during this process. Therefore it seems interresting to have a good Galois theory of q-difference equation for q equal to a root of unity. Because of the increasing size of the constant field at these points, such construction has been avoided for a long time. Recently P.Hendricks has proposed a solution to this problems but his Galois groups were defined over very transcendant fields. We propose here a new approach based, in a certain sense, on a q-deformation of the work of B.H. Matzat and Marius van der Put for Differential Galois theory in positive characteristic. We consider also a family of \textit{iterative difference operator} instead of considering, just one difference operator, and by this way we stop the increasing of the constant field and succeed to set up a Picard-Vessiot Theory for q-difference equations where q is a root of unity and relate it to a Tannakian approach.


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