1. B. D. Ardent and C. J. Stuth, On partial homomorphisms of semigroups, Pacific J. Math. 35, No. 1, (1970), 7–9.
2. C. J. Ash, Pseudovarieties, generalized varieties and similarly described classes, J. Algebra 92 (1985), 104–115.
3. S. Bogdanović, B. Imreh, M. Ćirić and T. Petković, Directable automata and their generalizations (A survey), in: S. Crvenković and I. Dolinka (eds.), Proc. VIII Int. Conf. “Algebra and Logic” (Novi Sad, 1998), Novi Sad J. Math. 29 (2) (1999), 31–74.
4. S. Burris and H. P. Sankappanavar, A course in universal algebra, Springer-Verlag, New York, 1981.
5. J. Černý, Poznámka k homogénnym experimentom s konečnými automatmi, Mat.-fyz. čas. SAV 14 (1964), 208–216.