S. Ginsburg and H. G. Rice, Two Families of Languages Related to ALGOL, Journal of the ACM, vol.9, issue.3, pp.350-371, 1962.
DOI : 10.1145/321127.321132

A. Je?-z, Conjunctive grammars can generate non-regular unary languages, International Journal of Foundations of Computer Science, vol.19, pp.3-597, 2008.

A. Je?-z and A. Okhotin, Conjunctive grammars over a unary alphabet: undecidability and unbounded growth, Theory of Computing Systems

A. Je?-z and A. Okhotin, Complexity of solutions of equations over sets of natural numbers, STACS, 2008.

A. Je?-z and A. Okhotin, On the computational completeness of equations over sets of natural numbers, part II, pp.63-74, 2008.

M. Kunc, The power of commuting with finite sets of words, Theory of Computing Systems, pp.521-551, 2007.

E. L. Leiss, Unrestricted complementation in language equations over a one-letter alphabet, Theoretical Computer Science, vol.132, issue.1-2, pp.71-93, 1994.
DOI : 10.1016/0304-3975(94)90227-5

P. Mckenzie and K. Wagner, The complexity of membership problems for circuits over sets of natural numbers, Computational Complexity, vol.16, pp.3-211, 2007.

A. Okhotin, Conjunctive grammars, Journal of Automata, Languages and Combinatorics, vol.6, issue.4, pp.519-535, 2001.

A. Okhotin, Decision Problems for Language Equations with Boolean Operations, 2003.
DOI : 10.1007/3-540-45061-0_21

A. Okhotin, Unresolved systems of language equations: Expressive power and decision problems, Theoretical Computer Science, vol.349, issue.3, pp.283-308, 2005.
DOI : 10.1016/j.tcs.2005.07.038

A. Okhotin and O. Yakimova, Language Equations with Complementation, pp.420-432, 2006.
DOI : 10.1007/11779148_38

L. J. Stockmeyer and A. R. Meyer, Word problems requiring exponential time, STOC, vol.1973, pp.1-9
DOI : 10.1145/800125.804029

URL : http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.116.4618