D. Caucal, J. Czyzowicz, W. Fraczak, and W. , Rytter Efficient computation of throughput values of context-free languages, 12 th CIAA, 2007.

J. Esparza, S. Kiefer, and M. , Luttenberger On fixed point equations over commutative semirings, th STACS, pp.296-307, 2007.

M. Hopkins, D. Th, L. , I. , and G. Longo, Kozen Parikh's theorem in commutative Kleene algebra, pp.394-401, 1999.

D. Muller and P. , The theory of ends, pushdown automata, and second-order logic, Theoretical Computer Science, vol.37, pp.51-75, 1985.
DOI : 10.1016/0304-3975(85)90087-8

R. Parikh, On context-free languages, JACM 13-4, pp.570-581, 1966.

D. Pilling, Commutative regular equations and Parikh's theorem, J. London Math Soc 6-4, pp.663-666, 1973.
DOI : 10.1112/jlms/s2-6.4.663

URL : http://jlms.oxfordjournals.org/cgi/content/short/s2-6/4/663

W. Thomas, A short introduction to infinite automata, 5 th DLT 01, LNCS, vol.2295, pp.130-144, 2001.
DOI : 10.1007/3-540-46011-x_10