1. The Algebraic Theory of Semigroups;Clifford,1962
2. Automata;Eilenberg,1976
3. Algebraic theory of machines;Krohn;Trans. Amer. Math. Soc.,1965
4. Categories for the Working Mathematician;MacLane,1971
5. A homomorphism theorem for finite semigroups;Rhodes;Math. Systems Theory,1967