1. Stanley Burris, H.P.Sankappanavar, A First Course in Universal Algebra, free online addition, 2022.
2. John M. Howie, Fundamentals of Semigroup Theory, London Mathematical Society Monographs, New Series vol. 12, Oxford University Press, Oxford, 1996.
3. W. J. Gilbert, W. K. Nicholson, Modern Algebra with Applications, Second Edition, John Wiley & Sons, Inc., 2004.
4. M. Sipser, Introduction to the Theory of Computation, Second Edition, Thomson Course Technology, 2006.