sgslogic.net

Anchor text: Stephen G. Simpson's home page

Wikipedia: Reverse mathematics

Anchor text: Model Theory

Wikipedia: Löwenheim–Skolem theorem

Anchor text: The Hierarchy Based on the Jump Operator

Wikipedia: Hyperarithmetical theory

Anchor text: Home page at PSU

Wikipedia: Steve Simpson (mathematician)

Anchor text: An Incompleteness Theorem for β_n-Models

Wikipedia: Beta-model

Wikipedia: Mučnik reducibility

Wikipedia: Medvedev reducibility