Non-amenable finitely presented torsion-by-cyclic groups Alexander Yu. Ol’shanskii, Mark V. Sapir (2003) Publications Mathématiques de l'IHÉS
Note on a question of Reinhold Baer on pregroups Seymour Lipschutz, Wujie Shi (2000) Publications de l'Institut Mathématique
Note on a Question of Reinhold Baer on Pregroups II Anthony M. Gaglione, Seymour Lipschutz, Dennis Spellman (2012) Publications de l'Institut Mathématique
Note on generalizing pregroups. Lipschutz, Seymour (1989) Publications de l'Institut Mathématique. Nouvelle Série