John Fountain: Semigroups, Algorithms, Automata and Languages. Hrsg.: Gracinda M. S. Gomes. World Scientific, 2002, ISBN 978-981-277-688-4, An introduction to covers for semigroups, S.167–168 (google.com).preprint
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John Fountain: Semigroups, Algorithms, Automata and Languages. Hrsg.: Gracinda M. S. Gomes. World Scientific, 2002, ISBN 978-981-277-688-4, An introduction to covers for semigroups, S.167–168 (google.com).preprint
