1. P. Ďuris, J. Hromkovič, One-way simple multihead finite automata are not closed under concatenation, Theoret. Comput. Sci. 27 (1983) 121–125.
2. R.W. Floyd, Review 14,353 of 18, Comput. Rev. 9 (1968) 280.
3. S. Ginsburg, The Mathematical Theory of Context-Free Languages, McGraw-Hill, New York, 1966.
4. S. Ginsburg, E.H. Spanier, Bounded Algol-like languages, Trans. Amer. Math. Soc. 113 (1963) 333–368.
5. S. Ginsburg, E.H. Spanier, Bounded regular sets, Proc. Amer. Math. Soc. 17 (1966) 1043–1049.