Borchardt, Björn. “A Pumping Lemma and Decidability Problems for Recognizable Tree Series”. Acta Cybernetica 16, no. 4 (January 1, 2004): 509–544. Accessed March 30, 2026. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3640.