Dyck-based characterizations of Indexed Languages
  Indexed languages are a generalization of context-free languages and form a proper subset of context-sensitive languages. We propose to generalize to indexed languages several well known characterizations of context-free languages: namely, the characterization by rational transductions defined by Nivat, the Chomsky-Sch\"utzenberger theorem, and the logical characterization proved by Lautemann et al. 
Severine Fratani and El Makki Voundy
cs.FL