On the Generative Power of Two.Level Morphological Rules

Abstract

Koskenniemi's model of two-level morphology has been very influential in recent years, but definitions of the formalism have generally been phrased in terms of a compilation (sometimes left unspecified) into a form of finite-state transducers, or else have consisted of an informal outline of the intended interpretation of the rule-formalism itself. Analyses of the properties of the formalism have generally focussed on the transducer mechanism. It is, however, possible to give a fully formal definition of the original rule notation directly, in a way which reflects Koskenniemi's original informal characterisation and which does not depend directly on the notion of a transducer (although it must retain the essential nature of parts of the notation as being regular expressions). This re-formulation allows a proof that the ability of this formalism to charaeterise mappings between strings is more limited than that of arbitrary transducers.