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L. C. Eggan, Transition graphs and the star height of regular events,*Michigan Math. J.*
**10** (1963), 385–397.

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R. McNaughton, Techniques for manipulating regular expressions,*Systems and Computer Sciences* (J. F. Hart and S. Takasu, eds.), pp. 24–41, University of Toronto Press, Toronto, 1965.

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R. McNaughton, The loop complexity of regular events,*Information Sci.*
**1** (1969), 305–328.

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R. McNaughton, The loop complexity of pure-group events,*Information and Control*
**11** (1967), 167–176.

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R. McNaughton andH. Yamada, Regular expressions and state graphs for automata,*IRE Trans. Electronics Computers*
**EC-9** (1960), 39–57; also in*Sequential Machines: Selected Papers*, (E. F. Moore, ed.), pp. 157–174, Addison-Wesley, Reading, Mass., 1963.

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J. A. Brzozowski, Derivatives of regular expressions,*J. Assoc. Comput. Math.*
**11** (1964), 481–494.

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J. A. Brzozowski andRina Cohen, on decompositions of regular events,*J. Assoc. Comput. Mach.*
**16** (1969), 132–144.

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R. S. Cohen andJ. A. Brzozowski, General properties of star height of regular events,*J. Comput. System Sci.*
**4** (1970), 260–280.

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R. S. Cohen, Star height of certain families of regular events,*J. Comput. System Sci.*
**4** (1970), 281–297.

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R. S. Cohen, Rank-non-increasing transformations on transition graphs, to appear.

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R. S. Cohen, Cycle Rank of Transition Graphs and the Star Height of Regular Events, Ph.D. Dissertation, University of Ottawa, Ottawa, 1968.

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F. Dejean andM. P. Schützenberger, On a question of Eggan,*Information and Control*
**9** (1966), 23–25.

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R. S. Cohen andJ. A. Brzozowski, On the star height of regular events,*Proceedings of the Eighth Annual Symposium on Switching and Automata Theory*, pp. 265–279, Inst. Elec. Electronics Eng., New York, 1967.

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R. S. Cohen, Transition graphs and the star height problem,*Proceedings of the Ninth Annual Symposium on Switching and Automata Theory*, pp. 383–394, Inst. Elec. Electronics Eng., New York, 1968.

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M. O. Rabin andD. Scott, Finite Automata and their Decision Problems.*IBM J. Res. Develop.*
**3** (1959), 114–125.