Abstract
We investigate the problem of expressing an arbitrary recursively enumerable language L 0in terms of an internal contextual language. It has been shown previously that L 0is a gsm image of a language generated by an internal contextual grammar with finite selectors. We first prove several variants of this characterization of recursively enumerable languages. We then establish two entirely new characterizations in terms of internal contextual grammars with finite selectors, applying either (1) leftmost (or prefix) derivations, or (2) erased contexts. The characterization obtained in (2) is particularly simple because one-sided grammars are sufficient in this case.
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