Kleene 1955 / Spector 1955: extends arithmetic-hierarchy to ordinal-stages via Π^1_1-comprehension. Δ^1_1 = hyperarithmetic. Foundation of descriptive set theory.
Kleene 1955 / Spector 1955: extends arithmetic-hierarchy to ordinal-stages via Π^1_1-comprehension. Δ^1_1 = hyperarithmetic. Foundation of descriptive set theory.