Condensation lemma

Condensation lemma

In set theory, a branch of mathematics, the condensation lemma is a result about sets in the constructible universe.

It states that if Lα is a level of the constructible hierarchy, X is an elementary submodel of the model (L_\alpha,\in) then (X,\in) is isomorphic to some (L_\beta,\in), \beta\leq\alpha.

The lemma was formulated and proved by Kurt Gödel in his proof that the axiom of constructibility implies GCH.

References