1. Commutativity and generation of n-parameter semigroups of bounded linear operators;Abdelaziz;Houston J. Math.,1983
2. S. Arora, S. Sharda, On two parameter semigroup of operators, Lecture Notes in Mathematics, 1511, Proceedings of a Conference held in Memory of U.N. Singh, New Delhi, India, 2–6 August, 1990
3. E. Hille, R.S. Phillips, Functional Analysis and Semigroups, Am. Math. Soc. Colloq. Publi. 31, Providence, Rhode Island, 1957
4. Semigroups of Linear Operators and Applications to Partial Differential Equations;Pazy,1983
5. On the product of semigroups of operators;Trotter;Proc. Am. Math. Soc.,1959