On the equivalence between generating functions computed by memory transducers and enumerating functions produced by indexed grammars

RESUMO

We consider the sequences of natural integers that can be computed by a deterministic transducer, with input in a structure A, output in N. and with memory the set of stacks of stacks of A. We show that these sequences are, exactly, the counting sequences of formal languages generated by unambiguous context-free indexed grammars (equivalently, the counting sequences of derivation trees of arbitrary context-free indexed grammars), with indexes in A. This general theorem applies, notably, to the set of natural integers endowed with the operation -1 and the non-zero predicate, showing that the polynomial recurrences count exactly the index-languages (where the parameter used for counting is the index itself).