I'm beginning to learn cohomology for cyclic groups in preparation for use in the proofs of global class field theory (using ideal-theoretic arguments). I've seen the proof of the long exact sequence and of basic properties of the Herbrand quotient, and I've started to look through how these are used in the proofs of class field theory.

So far, all I can tell is that the cohomology groups are given by some random ad hoc modding out process, then we derive some random properties (like the long exact sequence), and then we compute things like $H^0(\mathrm{Gal}(L/K),I_{L})$, H^2(\mathrm{Gal}(L/K),I_{L})$, where$I_L$denotes the group of fractional ideals of a number field$L$, and it ust just happens to be something interesting for the study of class field theory such as$I_K/\mathrm{N}(I_L)$, where$L/K$is cyclic and$\mathrm{N}$denotes the ideal normjnorm. We then find that the cohomology groups are useful for streamlining the computations with various orders of indexes of groups. What I don't get is what the intuition is behind the definitions of these cohomology groups. I do know what cohomology is in a geometric setting (so I know examples where taking the kernel modulo the image is interesting), but that doesn't help hereI don't know why we take these particular kernels modulo these particular images. What is the intuition for why they are defined the way they are? Why should we expect that these cohomology groups so-defined have nice properties and help us with algebraic number theory? Right now, I just see theorem after theorem, I see the algebraic manipulation and diagram chasing that proves it, but I don't see a bigger picture. For context, if$A$is a$G$-module where$G$is cyclic and$\sigma$is a generator of$G$, then we define the endomorphisms$D=1+\sigma+\sigma^2+\cdots+\sigma^{|G|-1}$and$N=1-\sigma$of$A$, and then$H^0(G,A)=\mathrm{ker}(N)/\mathrm{im}(D)$and$H^1(G,A)=\mathrm{ker}(D)/\mathrm{im}(N)$. (EDIT: This Note that this is a slight modification of group cohomology, i.e. Tate cohomology, which the cohomology theory primarily used for Class Field Theory. Group cohomology is the same but with$H^0(G,A) = \mathrm{ker}(N)$.)mathrm{ker}(N)$. The advantage of Tate cohomology is that it is $2$-periodic for $G$ cyclic.

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I'm beginning to learn cohomology for cyclic groups in preparation for use in the proofs of global class field theory (using ideal-theoretic arguments). I've seen the proof of the long exact sequence and of basic properties of the Herbrand quotient, and I've started to look through how these are used in the proofs of class field theory. So far, all I can tell is that the cohomology groups are given by some random modding out process, then we derive some random properties (like the long exact sequence), and then we compute things like $H^0(I_{L},\mathrm{Gal}(L/K))$, H^0(\mathrm{Gal}(L/K),I_{L})$, where$I_L$denotes the group of fractional ideals of a number field$L$, and it ust happens to be something interesting for the study of class field theory such as$I_K/\mathrm{N}(I_L)$, where$L/K$is cyclic and$\mathrm{N}$denotes the ideal normj. We then find that the cohomology groups are useful for streamlining the computations with various orders of indexes of groups. What I don't get is what the intuition is behind the definitions of these cohomology groups. I do know what cohomology is in a geometric setting (so I know examples where taking the kernel modulo the image is interesting), but that doesn't help here. What is the intuition for why they are defined the way they are? Why should we expect that these cohomology groups so-defined have nice properties and help us with algebraic number theory? Right now, I just see theorem after theorem, I see the algebraic manipulation and diagram chasing that proves it, but I don't see a bigger picture. For context, if$A$is a$G$-module where$G$is cyclic and$\sigma$is a generator of$G$, then we define the endomorphisms$D=1+\sigma+\sigma^2+\cdots+\sigma^{|G|-1}$and$N=1-\sigma$of$A$, and then$H^0(A)=\mathrm{ker}(N)/\mathrm{im}(D)$H^0(G,A)=\mathrm{ker}(N)/\mathrm{im}(D)$ and $H^1(A)=\mathrm{ker}(D)/\mathrm{im}(N)$. H^1(G,A)=\mathrm{ker}(D)/\mathrm{im}(N)$. (EDIT: This is a slight modification of group cohomology, i.e. Tate cohomology, which the cohomology theory primarily used for Class Field Theory. Group cohomology is the same but with$H^0(A) H^0(G,A) = \mathrm{ker}(N)$.) 5 improved formatting; deleted 155 characters in body; added 18 characters in body; added 22 characters in body I'm beginning to learn cohomology for cyclic groups in preparation for use in the proofs of global class field theory (using ideals)ideal-theoretic arguments). I've seen the proof of the long exact sequence and of basic properties of the Herbrand quotient, and I've started to look through how it's these are used in the proofs of class field theory. So far, all I can tell is that the cohomology groups are given by some random modding out process, and then we derive some random properties (like the long exact sequence), and then we compute things like$H^0(I_{L},\mathrm{Gal}(L/K))$, where$I_L$denotes the group of fractional ideals of a number field L,$L$, and it ust happens to be something interesting for the study of class field theory such as$(I_K/N(I_L))$, I_K/\mathrm{N}(I_L)$, where $L/K$ is cyclic and $N$ \mathrm{N}$denotes the ideal norm)normj. We then find that the cohomology groups are useful for streamlining the computations with various orders of indexes of groups. What I don't get is what the intuition is behind the definitions of these cohomology groups. I do know what cohomology is in a geometric setting (so I know examples where taking the kernel mod modulo the image is interesting), but that doesn't help here. What is the intuition for why they are defined the way they are? Why should we expect that these cohomology groups so-defined have nice properties and help us with algebraic number theory? Right now, I just see theorem after theorem, I see the algebraic manipulation and diagram chasing that proves it, but I don't see a bigger picture. For context, if A$A$is a G-module$G$-module where G$G$is cyclic and$\sigma$is the a generator of G,$G$, then we define the endomorphisms$D=1+\sigma+\sigma^2+...$D=1+\sigma+\sigma^2+\cdots+\sigma^{|G|-1}$ and $N=1-\sigma N=1-\sigma$ of A$,$A$, and then$H^0=kerN/imD$H^0(A)=\mathrm{ker}(N)/\mathrm{im}(D)$ and $H^1=kerD/imN$. H^1(A)=\mathrm{ker}(D)/\mathrm{im}(N)$. (EDIT: This is a slight modification of group cohomology, i.e. Tate cohomology, which the cohomology theory primarily used for Class Field Theory. Group cohomology is the same but with$H^0 H^0(A) = kerN$.) Edit: The thread http://mathoverflow.net/questions/8599/tips-on-cohomology-for-number-theory posted be Leonid Postiselski is very pertinent and useful!\mathrm{ker}(N)$.)

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