Multi-letter quantum finite automata: decidability of the equivalence and minimization of states

@article{Qiu2011MultiletterQF,
  title={Multi-letter quantum finite automata: decidability of the equivalence and minimization of states},
  author={Daowen Qiu and Lvzhou Li and Xiangfu Zou and Paulo Mateus and Jozef Gruska},
  journal={Acta Informatica},
  year={2011},
  volume={48},
  pages={271-290}
}
Multi-letter quantum finite automata (QFAs) can be thought of quantum variants of the one-way multi-head finite automata (Hromkovič, Acta Informatica 19:377–384, 1983). It has been shown that this new one-way QFAs (multi-letter QFAs) can accept with no error some regular languages, for example (a + b)*b, that are not acceptable by QFAs of Moore and Crutchfield (Theor Comput Sci 237:275–306, 2000) as well as Kondacs and Watrous (66–75, 1997; Observe that 1-letter QFAs are exactly measure-once… 
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