- S. Bosch, W. Lütkebohmert, M. Raynaud
The aim here is simply to provide some details to some of the proofs in Tate's paper [T]. 2. Tate's Section 2.2 2.1. Lemmas about divisibility. We say Γ → Γ is an isogeny of the formal group Γ = Spf(A) if the corresponding map A → A is injective and makes A free over itself of finite rank. Tate calls Γ divisible, if p : Γ → Γ is an isogeny. This is equivalent to ψ : A → A is injective and makes A ψ free of finite rank over A.