The equality S1 = D = R

The new result of this paper is that for θ( x ; a )‐stable (a weakening of “T is stable”) we have S1θ( x ; a )] = Dθ( x ; a ), L, ∞]. S1 is Hrushovski's rank. This is an improvement of a result of Kim and Pillay, who for simple theories under the (strong) assumption that either of the ranks be finite obtained the same identity. Only the first equality is new, the second equality is a result of Shelah from the seventies. We derive it by studying localizations of several rank functions, we get the following Main Theorem. Suppose that μ is regular satisfying μ ≥ |T|⁺, p is a finite type, and Δ is a set of formulas closed under Boolean operations. If either (a) Rp, Δ, μ⁺] < ∞ or (b) p is Δ‐stable and μ satisfies “for every sequence {μ_i : i < |Δ| + ?₀} of cardinals μ_i < μ we have that holds”, then Sp, Δ, μ⁺] = Dp, Δ, μ⁺] = Rp, Δ, μ⁺]. The S rank above is a localized version of Hrushovski's S1 rank. This rank, as well as our systematic use of local stability, allows us to get a more conceptual proof of the equality of D and R, which is an old result of Shelah. A particular (asymptotic) case of the theorem offers a new sufficient condition for the equality of S1 and D·, L, ∞]. We also manage, due to a more general approach, to avoid some combinatorial difficulties present in Shelah's original exposition.