Makoto Sakamoto and  Yasuo Uchida and  Makoto Nagatomo and  Takao Ito and Tsunehiro Yoshinaga and Satoshi Ikeda and Masahiro Yokomichi and  Hiroshi Furutani,  Hierarchies Based On the Number of Cooperating Systems of Finite Automata on Four-Dimensional Input Tapes.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    December 2012, vol. 72(12). 1707 - 1712
    [viewed 20 September 2020]. Available from: https://publications.waset.org/pdf/6351.