- Andrzej Szepietowski:
There is no complete axiom system for shuffle expressions. RAIRO Theor. Informatics Appl. 33(3): 271-278 (1999) - Andrzej Szepietowski:
Lower space bounds for accepting shuffle languages. RAIRO Theor. Informatics Appl. 33(3): 303-307 (1999)