[1] C.K. Lai and K.P. Shum: 
Characterizations of dual semigroups. Czechoslovak Math. J. 39 (1989), 518–584. 
MR 1017989 
[2] L. Márkl: 
Structure theorems on certain regular and inverse semigroups. Czechoslovak Math. J. 27 (1977), 388–393. 
MR 0444816 
[4] St. Schwarz: 
On the structure of dual semigroups. Czechoslovak Math. J. 21 (1971), 461–483. 
MR 0292982 | 
Zbl 0232.20116