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C U E ~ O€C = O} ~ = /oSC~ n , ~ EaeCr / ~ n . = /O€C o /~ = ~ 1-5-1. = ~ 1 ( ~ - i ) I. 28 COHOMOLOGY GROUPS OF G IN A Proof: An n-cochain (with respect to the standard complex X = X8) is completely determined by its values on the n-cells. PI -fb,TI d 7 , Vo, T , p E G Thus, f is a cocycle if and only if it satisfies Furthermore, the 2-cochain f is a coboundary if and only if it is of the form f = Sg for some g E %“-thus, it must satisfy Functions of two variables satisfying the cocycle identity were known classically as factor sets, while those satisfying the coboundary identity were known as splitting factor sets.

Hilbert's theorem 90). If a E K has NK+Fa= 1, then there exists /? l-". Define a 1-cocycle f :G Proof: f(1) 1-54. , = a,f(u2) = ~(uoL),f(un-') = a(ua) Proposition. a * * (u"-*a) I Ho(G,A ) m AG/(SA). ] onto A. Given f E P we have is an isomorphism of '30 1-5. ] COHOMOLOGY GROUPS OF G IN A 31 = u, then f is a 0-cocycle t> (u - 1) u - =0 Vu E G and our mapping induces an isomorphism of Toonto AG. ) is an isomorphism of Furthermore, the mapping g V-' onto A. )) and the map onto SA. I aESA f-j[*] also induces an isomorphism of 990 1-5-7.

34 (n COHOMOLOGY GROUPS OF G I N A For each n 2 0 consider functions @ which map ordered 1)-tuples of elements of G into A-that is, + @ : Gx * - a x G-A. ,on E G are called n-dimensional homogeneous cochains; they form a group denoted by an = P ( G , A). *-,un+l) - where $$ indicates that this term is omitted. 6 (which is called the and it coboundary) is clearly a homomorphism of 6" (called the group satisfies 8 0 8 = 0. Denote the kernel of 6 by 3% of n-cocyctes) and the image of 6 by b"+l(called the group of (n + 1)-coboundaries).

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