Four notions of conjugacy for abstract semigroups
          Araújo, João; Kinyon, M.; Konieczny, J.; Martins, Nuno F. M.  
          
          Proceedings of the Royal Society of Edinburgh Section A: Mathematics, volume 147, issue 6 (2017), 1169-1214  
          https://doi.org/10.1017/S0308210517000099  
           
            
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