Obstruction sets and extensions of groups
Let X be a nice variety over a number field k. We characterise in pure “descent-type” terms some inequivalent obstruction sets refining the inclusion . In the first part, we apply ideas from the proof of by Skorobogatov and Demarche to new cases, by proving a comparison theorem for obstruction sets. In the second part, we show that if are such that , then . This allows us to conclude, among other things, that and .