Valuations and differential Galois groups by Guillaume Duval

By Guillaume Duval

During this paper, valuation thought is used to examine infinitesimal behaviour of strategies of linear differential equations. For any Picard-Vessiot extension $(F / okay, \partial)$ with differential Galois team $G$, the writer seems on the valuations of $F$ that are left invariant via $G$. the most reason behind this can be the subsequent: If a given invariant valuation $\nu$ measures infinitesimal behaviour of features belonging to $F$, then conjugate components of $F$ will proportion an analogous infinitesimal behaviour with admire to $\nu$. This memoir is split into seven sections

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G g It is therefore obvious that the same rule occurs in H . Now we check that Γvˆ = Γv . From the identities Γv = H∗ /U(Rv ) and Γvˆ = ∗ H /U(Rvˆ ), we have a natural injection Γv → Γvˆ , and it still needs to be shown that this map is onto. Let s = f + ig ∈ H∗ if f × g = 0, then the image of s in Γvˆ belongs to Im(Γv → Γvˆ ). If f × g = 0, since s and is have the same image in Γvˆ , we may assume that v(f ) v(g). Setting s = gu with u = f /g + i, limu ∈ ∗ hence u ∈ U(Rvˆ ). So the image of s in Γvˆ is the same as the one of g.

F ∂f ∂X (α) ∂X (α) ∂X (α) satisfies condition (c). This ends the proof of Lemma 46, as well as the one of the first step. 2. Henselian rings, proof of the second step. We briefly recall some results from the Theory of Henselian rings, as introduced in [18]. One of the equivalent definitions of the Henselian rings is Azumaya’s separation criterion (see [18], def 1, p. 1). Definition 47. Let (A, m) be a local domain. A is an Henselian ring if every finite A-algebra B splits into a finite product of local A-algebras.

Also by ∂. Let x0 Proof. Let’s denote the extension of the derivation of K to K be in K ∗ such that, ∂x ∀x ∈ K ∗ , ν ν(x0 ) = ω0 . x We have the following inclusion ∂(A) ⊂ x0 · A ⊂ x0 · A˜ = M. 44 GUILLAUME DUVAL ˜ we have: By Lemma 50, for any finite ´etale A-subalgebra B of A, ∂(B) ⊂ x0 · A˜ = M. Set P = B ∩ m. ˜ Then: ∂(BP ) ⊂ (B\P)−1 · ∂(B) ⊂ x0 · A˜ = M. By Theorem 48, A˜ coincides with the union of such BP , hence ˜ ˜ ⊂ x0 · A. ∂(A) ˜ ν(∂(u)) ˜ ⊂ Lν (ω0 )). Therefore, for any unit u ∈ A, ν(x0 ) = ω0 , (that is U(A) ˜ Since K/K is algebraic, Γν˜ /Γν is an Abelian torsion group.

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