Kahn's concrete

Kahn's concrete refers to a mathematical concept from Kahn’s theorem in computability theory, which deals with the complexity of enumerable problems. It describes a set of problems or functions that can be approximated from below by a sequence of increasingly better solutions, but no single solution captures the entire problem at once. These sets are "maximal" in the sense that they contain as many problems as possible with this property, meaning they are not contained within any larger class of similar problems. Kahn's concrete provides a way to understand the limitations and structure of computable processes and problem-solving in theoretical computer science.