Mathematical Institute, University of Gdańsk, ul. Wita Stwosza 57, 80-952 Gdańsk, Po land; firstname.lastname@example.org.
In this paper we show that neither the set of all valid equations between shuffle expressions nor the set of schemas of valid equations is recursively enumerable. Thus, neither of the sets can be recursively generated by any axiom system.
(Received January 1999)
(Accepted March 1999)
(Online publication August 15 2002)
Mathematics Subject Classification: