His paper Embedding theorems for groups written jointly with both Bernhard Neumann, who as we noted was a colleague of Graham Higman's at Manchester at that time, and with Hanna Neumann, introduces the now standard group construction of HNN extensions (Higman - Neumann - Neumann extensions).
Kanyang papel embed theorems para sa grupo ng nakasulat na sama-sama sa parehong Bernhard Neumann, na kilala bilang namin ay isang kasamahan ng Graham Higman's at Manchester sa oras na iyon, at sa Hanna Neumann, introduces sa ngayon standard na grupo ng konstruksyon ng HNN extension (Higman - Neumann - Neumann extension).