Composition Closure of Linear Extended Top-down Tree Transducers


Linear extended top-down tree transducers (or synchronous tree-substitution grammars) are popular formal models of tree transformations that are extensively used in syntax-based statistical machine translation. The expressive power of compositions of such transducers with and without regular look-ahead is investigated. In particular, the restrictions of ε-freeness, strictness, and nondeletion are considered. The composition hierarchy turns out to be finite for all ε-free (all rules consume input) variants of these transducers except for the nondeleting ε-free transducers. The least number of transducers needed for the full expressive power of arbitrary compositions is presented. In all remaining cases (incl. the nondeleting ε-free transducers) the composition hierarchy does not collapse.

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Correspondence to Andreas Maletti.

Additional information

This is a revised and extended version of [Z. Fülöp and A. Maletti: Composition closure of ε -free linear extended top-down tree transducers. In Proc. 17th DLT, volume 7907 of LNCS, pages 239–251. Springer-Verlag, 2013].

This work was partially supported by the exchange project 55 657 of the German Academic Exchange Service (DAAD) and Hungarian Scholarship Board Office (MÖB). Z. Fülöp was partially supported by the NKFI grant K 108 448, and A. Maletti was partially supported by the German Research Foundation (DFG) grant MA / 4959 / 1-1.

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Engelfriet, J., Fülöp, Z. & Maletti, A. Composition Closure of Linear Extended Top-down Tree Transducers. Theory Comput Syst 60, 129–171 (2017).

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  • Extended top-down tree transducer
  • Composition hierarchy
  • Bimorphism