I am interested in the action of field automorphism group $Aut(\mathbb{C}/\mathbb{Q})$ on $Ext^1(\mathbb{C}^{\star},\mathbb{Z})$ or $Ext^1(\mathbb{\bar Q}^*,\mathbb{Z})$, and, more generally, on $Ext^1_{EndA}(A(\mathbb{C}),\pi_1(A(\mathbb{C},0))/Aut_{EndA}(\pi_1(A(\mathbb{C},0) )$ for $A$ a commutative algebraic group. I am wondering whether this kind of questions have been considered in algebraic geometry, e.g. in relation to the full algebraic fundamental group of A(C) or $A(\mathbb{\bar Q})$ as a scheme over $\mathbb Q$.

These have been studied in logic (see http://arxiv.org/abs/math/0511591, http://arxiv.org/abs/0704.3561 , and also this ), and are related to questions of arithmetics, such as Galois action on a Tate module.

The main result of these papers is the following; from the point of view of logic it is a categoricity theorem. First note that $ exp :\mathbb{C}\rightarrow \mathbb{C}^\star$ gives an element of $Ext^1(\mathbb{C}^{\star},\mathbb{Z})$ (and also $Ext^1(\mathbb{\bar Q}^*,\mathbb{Z})$). Then the orbit of this extensions consists of all extensions where the middle is an $\mathbb{Q}$-vector space. A similar result is known about the covers of elliptic curves in char 0. Also, I may mention that there are partial analogues in prime characteristic.

  • $\begingroup$ What exactly do you mean by "algebraic" fundamental group? The etale fundamental group? Or the Schur multiplier (second homology with integer coefficients)? $\endgroup$ – Konrad Voelkel May 12 '11 at 20:56
  • $\begingroup$ the etale fundamental group $\pi_1^{et}(A\tensor C,0)$ where $A$ is my algebraic group. $\endgroup$ – mmm Jun 14 '11 at 14:28

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