Extension theorem may refer to:Carathéodory's extension theorem - a theorem in measure theory, named after the Greek mathematician Constantin Carathéodory Dugundji extension theorem - a theorem in topology, named after the American mathematician James Dugundji Extension Lemma - a lemma in topology, related to the Tietze extension theorem Hartogs' extension theorem - a theorem in the theory of functions of several complex variables Isomorphism extension theorem - a theorem in field theory Kolmogorov extension theorem - a theorem in probability theory, named after the Soviet mathematician Andrey Nikolaevich Kolmogorov Krein extension theorem - a theorem in functional analysis, proved by the Soviet mathematician Mark Grigorievich Krein M. Riesz extension theorem - a theorem in mathematics, proved by Marcel Riesz Ohsawa–Takegoshi L2 extension theorem - a theorem in several complex variables, proved by Takeo Ohsawa and Kensho Takegoshi Szpilrajn extension theorem - a theorem in set theory, proved by Edward Szpilrajn Tietze extension theorem - a theorem in topology, named after the Austrian mathematician Heinrich Franz Friedrich Tietze Urysohn extension theorem - a theorem in topology, named after the mathematician Pavel Samuilovich Urysohn Whitney extension theorem - a theorem in mathematical analysis, named after the American mathematician Hassler Whitney Kreps extension theorem - a theorem in financial mathematics, ensuring that incomplete markets can be completed by adding the right securities