Colliot-Thélène's approximation conjecture for zero-cycles

Let XX be a smooth, projective, geometrically integral variety of dimension dd over a number field kk. For each place vv of kk, write Xv=X×kkvX_v=X\times_k k_v, let Z0(Xv)Z_0(X_v) be the group of zero-cycles, and let Ωk\Omega_k be the set of places of kk. For an integer δ\delta, a family {zv}vΩkZ0(Xv)\{z_v\}\in\prod_{v\in\Omega_k}Z_0(X_v) has degree δ\delta and is orthogonal to the Brauer group Br(X)Br(X) when its Brauer–Manin pairing with every element of Br(X)Br(X) is zero. Let H~2d(Xv,Z/mZ(d))\widetilde{H}^{2d}(X_v,\mathbb{Z}/m\mathbb{Z}(d)) be the modified étale cohomology group appearing in the local cycle class map.

Colliot-Thélène's approximation conjecture. If {zv}vΩkZ0(Xv)\{z_v\}\in\prod_{v\in\Omega_k}Z_0(X_v) is a degree-δ\delta family orthogonal to Br(X)Br(X), then for every positive integer mm there exists a global zero-cycle zmZ0(X)z_m\in Z_0(X) of degree δ\delta such that, at every place vv, the cycles zmz_m and zvz_v have the same image under

CH0(Xv)H~2d(Xv,Z/mZ(d)).CH_0(X_v)\to\widetilde{H}^{2d}(X_v,\mathbb{Z}/m\mathbb{Z}(d)).

This is a stronger approximation statement than mere existence of a global zero-cycle: it requires simultaneous congruence in the local cycle-class groups for every modulus mm. The supplied source attributes the conjectural statements to Colliot-Thélène; no resolution status is given here.

Sources & referencesView supporting material

Primary source

Yongqi Liang, “Principe local-global pour les zéro-cycles sur certaines fibrations au-dessus d'une courbe : I”, arXiv:1006.2572 (2011).

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