In mathematics, the HNN extension is an important construction of combinatorial group theory.
Introduced in a 1949 paper Embedding Theorems for Groups[1] by Graham Higman, Bernhard Neumann, and Hanna Neumann, it embeds a given group G into another group G' , in such a way that two given isomorphic subgroups of G are conjugate (through a given isomorphism) in G' .
- ^ Higman, Graham; Neumann, Bernhard H.; Neumann, Hanna (1949). "Embedding Theorems for Groups". Journal of the London Mathematical Society. s1-24 (4): 247–254. doi:10.1112/jlms/s1-24.4.247.