no. 1
Two-way multihead automata over a one-letter alphabet
RAIRO. Informatique théorique, Tome 14 (1980) no. 1, pp. 67-82. 
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     volume = {14},
     number = {1},
     year = {1980},
     mrnumber = {570039},
     zbl = {0442.68039},
     language = {en},
     url = {http://archive.numdam.org/item/ITA_1980__14_1_67_0/}
}
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Monien, Burkhard. Two-way multihead automata over a one-letter alphabet. RAIRO. Informatique théorique, Tome 14 (1980) no. 1, pp. 67-82. http://archive.numdam.org/item/ITA_1980__14_1_67_0/

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