no. 2
Undecidability of the equivalence of finite substitutions on regular language
RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 33 (1999) no. 2, pp. 117-124. 
@article{ITA_1999__33_2_117_0,
     author = {Halava, Vesa and Harju, Tero},
     title = {Undecidability of the equivalence of finite substitutions on regular language},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
     pages = {117--124},
     publisher = {EDP-Sciences},
     volume = {33},
     number = {2},
     year = {1999},
     mrnumber = {1707965},
     zbl = {0946.68076},
     language = {en},
     url = {http://www.numdam.org/item/ITA_1999__33_2_117_0/}
}
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Halava, Vesa; Harju, Tero. Undecidability of the equivalence of finite substitutions on regular language. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 33 (1999) no. 2, pp. 117-124. http://www.numdam.org/item/ITA_1999__33_2_117_0/

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