fernau@ira.uka.de (Henning Fernau) (04/25/91)
Dear netters! I should appreciate your help in the following matter: I (well, we all) know that there is a huge amount of literature on word relations of the form $L\subset\Sigma^{\ast}$, i.e. on the `classical' formal languages. Alas, I know only _very_ few articles dealing with relations of the form $R\subset(\Sigma^\ast)^n$ with $n\gt 1$. If you know anything in this area, would you please send pointers to literature etc.? Are there any accepted hierarchies of families of word relations like the Chomsky hierarchy in language theory? The same question arises when you consider $\omega$-languages (or relations, respectively). If there is a considerable interest in the net community, I should like to post a summary of the received answers. Thanks in advance, Henning Fernau Lehrstuhl Informatik fuer Ingenieure und Naturwissenschaftler University of Karlsruhe Am Fasanengarten 5 W-7500 Karlsruhe 1 phone: [West Germany](0721)608-4336 email:fernau@ira.uka.de