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Distributional Learning of Some Nonlinear Tree Grammars

Clark A., Kobele G. M., Kanazawa M., Yoshinaka R.

Fundamenta Informaticae

2016

Vol. 146, nr 4

339--377

A key component of Clark and Yoshinaka’s distributional learning algorithms is the extraction of substructures and contexts contained in the input data. This problem often becomes intractable with nonlinear grammar formalisms due to the fact that more than polynomially many substructures and/or contexts may be contained in each object. Previous works on distributional learning of nonlinear grammars avoided this difficulty by restricting the substructures or contexts that are made available to the learner. In this paper, we identify two classes of nonlinear tree grammars for which the extraction of substructures and contexts can be performed in polynomial time, and which, consequently, admit successful distributional learning in its unmodified, original form.

Some Alternatives to Parikh Matrices Using String Kernels

Clark A., Watkins C.

Fundamenta Informaticae

2008

Vol. 84, nr 3-4

291-303

We describe methods of representing strings as real valued vectors or matrices; we show how to integrate two separate lines of enquiry: string kernels, developed in machine learning, and Parikh matrices [8], which have been studied intensively over the last few years as a powerful tool in the study of combinatorics over words. In the field of machine learning, there is widespread use of string kernels, which use analogous mappings into high dimensional feature spaces based on the occurrences of subwords or factors. In this paper we show how one can use string kernels to construct two alternatives to Parikh matrices, that overcome some of the limitations of the Parikh matrix construction. These are morphisms from the free monoid to rings of real-valued matrices under multiplication: one is based on the subsequence kernel and the other on the gap-weighted string kernel. For the latter kernel we demonstrate that for many values of the gap-weight hyper-parameter the resulting morphism is injective.