1. B. Ravikumar, Approximations by regular languages, 2017, submitted for publication.
2. Thin and slender languages;Păun;Discrete Appl. Math.,1995
3. On lengths of words in context-free languages;Ilie;Theoret. Comput. Sci.,2000
4. Slender matrix languages;Stiebe,2000
5. Sur la densite de langages formels;Berstel,1972