By Grégory Berhuy

This e-book is the 1st ordinary creation to Galois cohomology and its functions. the 1st half is self contained and offers the elemental result of the speculation, together with a close development of the Galois cohomology functor, in addition to an exposition of the overall conception of Galois descent. the complete conception is influenced and illustrated utilizing the instance of the descent challenge of conjugacy periods of matrices. the second one a part of the booklet offers an perception of the way Galois cohomology might be precious to unravel a few algebraic difficulties in different lively learn themes, similar to inverse Galois concept, rationality questions or crucial size of algebraic teams. the writer assumes just a minimum historical past in algebra (Galois conception, tensor items of vectors areas and algebras).

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17. 36 Cohomology of proﬁnite groups (1) In the existing literature, n-cocycles and n-coboundaries are often deﬁned to be elements of ker(dn ) and im(dn−1 ) respectively, and cocycles and coboundaries satisfying the extra condition (2) are called normalized. However, one can show that the two quotient groups obtained with these two diﬀerent deﬁnitions are canonically isomorphic. (2) If Γ is a ﬁnite abstract group, we recover the classical deﬁnition of the cohomology groups associated to a ﬁnite group (modulo the previous remark).

We refer the reader to [12] for more details. A proﬁnite group Γ is compact and totally disconnected (that is the only non-empty connected subsets are one-point subsets). In particular, one-point subsets are closed, every open subgroup is also closed and has ﬁnite index. Moreover, every neighbourhood of 1 contains an open normal subgroup (hence of ﬁnite index). If Γ is a closed subgroup of Γ, then Γ is proﬁnite, and if moreover Γ is normal, so is Γ/Γ . Finally, if N denotes the set of open normal subgroups of Γ, the map θ: Γ −→ lim ←− Γ/U U ∈N g −→ (gU )U is an isomorphism of topological groups.

If [α] ∈ H n (Γ/U, AU ), the cohomology class inf U,U ([α]) is represented by the cocycle β: (Γ/U )n −→ AU (πU (σ1 ), . . ,πU (σn ) . 25. Let Γ be a proﬁnite group acting continuously on A, and let U ∈ N be a normal open subgroup of Γ. Then the maps πU : Γ −→ Γ/U and the inclusion AU −→ A are compatible, and give rise to a map fU : H n (Γ/U, AU ) −→ H n (Γ, A). 26. For i = 1, . . , 4, let Ai be a Γi -set (Γi -group,etc). Assume that we have two commutative diagrams A1 f1 f3 A4 / A2 f2 f4 / A3 and ΓO 1 o ϕ1 ϕ3 Γ4 o ΓO 2 ϕ2 ϕ4 Γ3 where ϕi is a morphism of proﬁnite groups compatible with fi , for i = 1, .