1. Berstel, J. [1979]. “Transductions and Context free languages”, Teubner Studienbücher, Stuttgart.
2. Boasson, L. [1971]. “Cones rationnels et familles agreables de langages — application aux langages a compteur”. Thèse de 3ème cycle. Paris VII.
3. Boasson, L; B. Courcelle; M. Nivat [1981]. “The rational index a complexity measure for languages”. SIAM Journal on computing 10, 2, 284–296.
4. Book, R. [1982]. “Intersection of CFL's and related structures” in Actes de l'école de printemps de théorie de langages. Editeur Blab. M. publication of LITP nℴ 82-14.
5. Bucher. W; K. Culik; H. Maurer; D. Wotschke. “Concise description of finite languages”. Theor. Comput. Sci. 14, 3, 211–347 (1981).