A finer reparameterisation theorem for MSO and FO queries on strings
Provides a theoretical foundation for understanding the structure of MSO and FO queries on strings, relevant to database theory and formal language theory.
The paper proves a reparameterisation theorem for MSO and FO queries on strings, showing that if a query's result count is bounded by a product of symbol counts, each result can be identified from a few positions and finite data. This yields a proof of dimension minimisation for first-order string-to-string interpretations.
We show a theorem on monadic second-order k-ary queries on finite words. It may be illustrated by the following example: if the number of results of a query on binary strings is O(number of 0s $\times$ number of 1s), then each result can be MSO-definably identified from a 0-position, a 1-position and some finite data. Our proofs also handle the case of first-order logic / aperiodic monoids. Thus we can state and prove the folklore theorem that dimension minimisation holds for first-order string-to-string interpretations.