This chapter is devoted to the study of certain Galois representations. In the first section we introduce and study Mazur's deformation theory and discuss various refinements of it. These refinements will be needed later to make precise the correspondence between the universal deformation rings and the Hecke rings in Chapter 2. The main results needed are Proposition 1.2 which is used to interpret various generalized cotangent spaces as Selmer groups and (1.7) which later will be used to study them. At the end of the section we relate these Selmer groups to ones used in the Bloch—Kato conjecture, but this connection is not needed for the proofs of our main results.
In the second section we extract from the results of Poitou and Tate on Galois cohomology certain general relations between Selmer groups as