1. E.Cardoza, R.Lipton and A.R.Meyer: "Exponential Space Complete Problem for Petri Nets and Commutative Semigroups", in "Proc. of the 8-th Annual ACM Symposium on the Theory of Computing" (1976), pp. 50–54.
2. S.Eilenberg and M.P.Schützenberger: "Rational Sets in Commutative Monoids", Journal of Algebra, No 13, 1969, pp. 173–191.
3. J. Von Zur Gathen and M. Sieveking: "A bound on Solutions of Linear Integer Equalities and Inequalities", Proc. of the AMS, Vol. 72, No 1, 1978, pp. 155–158.
4. M. Gerstenhaber: "Theory of Convex Polyhedral Cones", in Proc. of a Conference on "Activity Analysis of Production and Allocation", Ed. T.C. Koopmans, John Willey & Sons — Chapman & Hall, 1951.
5. S.Ginsburg: "The Mathematical Theory of Context-free Languages", Mc Graw-Hill, 1966.