[1] C. L. Adair: Bands with involution. J. Algebra 75 (1982), 297–314. DOI 10.1016/0021-8693(82)90042-4 | MR 0653894

[2] K. Auinger: Bifree objects in $e$-varieties of strict orthodox semigroups and the lattice of strict orthodox $*$-semigroup varieties. Glasgow Math. J. (to appear). MR 1199935

[3] K. Auinger: Strict regular $*$-semigroups. Proceedings of the 1991 Oberwolfach Semigroup Conference (to appear). MR 1197855

[4] A. H. Clifford: The free completely regular semigroup on a set. J. Algebra 59 (1979), 434–451. DOI 10.1016/0021-8693(79)90139-X | MR 0543262 | Zbl 0462.20047

[5] J. A. Gerhard and M. Petrich: Free bands and free $*$-bands. Glasgow Math. J. 28 (1986), 161–179. DOI 10.1017/S0017089500006480 | MR 0848423

[6] J. A. Gerhard and M. Petrich: Free involutorial completely simple semigroups. Can. J. Math. 37 (1985), 271–295. DOI 10.4153/CJM-1985-017-9 | MR 0780625

[7] T. E. Hall: Identities for existence varieties of regular semigroups. Bull. Austral. Math. Soc. 40 (1989), 59–77. DOI 10.1017/S000497270000349X | MR 1020841 | Zbl 0666.20028

[8] T. E. Hall: Regular semigroups: amalgamation and the lattice of existence varieties. Algebra Universalis 28 (1991), 79–108. DOI 10.1007/BF01190413 | MR 1083823 | Zbl 0727.20041

[9] J. M. Howie: An Introduction to Semigroup Theory. Academic Press, London, 1976. MR 0466355 | Zbl 0355.20056

[10] H. Mitsch: A natural partial order for semigroups. Proc. Amer. Math. Soc. 97 (1986), 384–388. DOI 10.1090/S0002-9939-1986-0840614-0 | MR 0840614 | Zbl 0596.06015

[11] K. S. S. Nambooripad: The natural partial order on a regular semigroup. Proc. Edinburgh Math. Soc. 23 (1980), 249–260. MR 0620922 | Zbl 0459.20054

[12] K. S. S. Nambooripad and F. Pastijn: Regular involution semigroups. Semigroups, Coll. Math. Soc. János Bolyai 39 (1986), 199–249. MR 0873153

[13] T. E. Nordahl and H. E. Scheiblich: Regular $*$-semigroups. Semigroup Forum 16 (1978), 369–377. DOI 10.1007/BF02194636 | MR 0507171

[14] F. Pastijn: The structure of pseudo-inverse semigroups. Trans. Amer. Math. Soc. 273 (1982), 631–655. DOI 10.1090/S0002-9947-1982-0667165-1 | MR 0667165 | Zbl 0512.20042

[15] F. Pastijn: Rectangular bands of inverse semigroups. Simon Stevin 56 (1982), 1–97. MR 0662981 | Zbl 0499.20038

[16] M. Petrich: Certain varieties of completely regular $*$-semigroups. Boll. Un. Mat. It. 4(B) (1985), 343–370. MR 0805417

[17] M. Petrich: Inverse Semigroups. Wiley, New York, 1984. MR 0752899 | Zbl 0546.20053

[18] L. Polák: Varieties of unary semigroups.

[19] B. Pondělíček: On varieties of regular $*$-semigroups. Czechoslovak Math. J. 41 (1991), 110–119. MR 1087630

[20] H. E. Scheiblich: Free inverse semigroups. Proc. Amer. Math. Soc. 38 (1973), 1–7. DOI 10.1090/S0002-9939-1973-0310093-1 | MR 0310093 | Zbl 0256.20079

[21] H. E. Scheiblich: Generalized inverse semigroups with involution. Rocky Mountain J. Math. 12 (1982), 205–211. DOI 10.1216/RMJ-1982-12-2-205 | MR 0661728 | Zbl 0504.20040

[22] B. M. Schein: Free inverse semigroups are not finitely presentable. Acta Math. Acad. Sci. Hung. 26 (1975), 41–52. DOI 10.1007/BF01895947 | MR 0360878 | Zbl 0301.20041

[23] M. B. Szendrei: Free $*$-orthodox semigroups. Simon Stevin 59 (1985), 175–201. MR 0805105

[24] M. B. Szendrei: A new interpretation of free orthodox and generalized inverse $*$-semigroups. Springer Lecture Notes 1320 (1986), 358–371. MR 0957780