@article{Green&Rees:1952,
	key = "Green and Rees 1952",
	author = "J. A. Green and D. Rees",
	title = "On semi-groups in which $x^r = x$",
	journal = PCPS,
	volume = 48,
	year = 1952,
	pages = "35-40"}

@book{Arbib:1968,
	key = "Arbib 1968",
	editor = "M. A. Arbib",
	title = "Algebraic Theory of Machines, Languages, and Semigroups",
	publisher = AP,
	year = 1968}

@book{Kuich&Salomaa:1986,
	key = "Kuich and Salomaa 1986",
	author = "W. Kuich and A. Salomaa",
	title = "Semirings, Automata, Languages",
	publisher = SV,
	year = 1986}

@incollection{Kuich:1997,
	key = "Kuich 1997",
	author = "W. Kuich",
	title = "Semirings and formal power series",
	booktitle = "Handbook of Formal Languages",
	editor = "G. Rozenberg and A. Salomaa",
	volume = "1",
	publisher = SV,
	year = 1997,
	pages = "609-677"}