Chomsky, Knowledge of Language and the Rule-Following Considerations

According to Noam Chomsky, speakers of a language have a substantial body of propositional knowledge of that language that they draw upon in language production and comprehension. Since the late 1950s Chomsky"s project has been to characterise that knowledge and give an account of its acquisition. Arguably, one of the most powerful philosophical challenges to Chomsky"s output is generated by the rule following considerations of Philosophical Investigations §§ 138-242. My aim in this paper is to characterise the nature of this challenge, a topic that, rather surprisingly, has received relatively little attention in the philosophical literature

Recent MAGSAT results

Meyer, et al. have improved their original global crustal model and made a spherical harmonic analysis of the resulting magnetic field to n=50. The Z contours at 400 Km altitude from a field model composed of the first 15 degrees and order of their model and the terms n=16 to 29 from the MAGSAT model M051782 are presented. The main point to consider from such representations is that the lower order terms appear to contribute components comparable in magnitude to those of higher order. Thus, one should allow in making tectonic interpretations of global maps of anomalies such as those published by Langel, that there are likely continental scale (or smaller) features that have been removed along with the core field by the subtraction of the terms n=1 to 13 of the observed field. Planning for the analysis of data to be accrued by GRM should thus address this problem

Crystallizing the hypoplactic monoid: from quasi-Kashiwara operators to the Robinson--Schensted--Knuth-type correspondence for quasi-ribbon tableaux

Crystal graphs, in the sense of Kashiwara, carry a natural monoid structure given by identifying words labelling vertices that appear in the same position of isomorphic components of the crystal. In the particular case of the crystal graph for the $q$-analogue of the special linear Lie algebra $\mathfrak{sl}_{n}$, this monoid is the celebrated plactic monoid, whose elements can be identified with Young tableaux. The crystal graph and the so-called Kashiwara operators interact beautifully with the combinatorics of Young tableaux and with the Robinson--Schensted--Knuth correspondence and so provide powerful combinatorial tools to work with them. This paper constructs an analogous `quasi-crystal' structure for the hypoplactic monoid, whose elements can be identified with quasi-ribbon tableaux and whose connection with the theory of quasi-symmetric functions echoes the connection of the plactic monoid with the theory of symmetric functions. This quasi-crystal structure and the associated quasi-Kashiwara operators are shown to interact just as neatly with the combinatorics of quasi-ribbon tableaux and with the hypoplactic version of the Robinson--Schensted--Knuth correspondence. A study is then made of the interaction of the crystal graph of the plactic monoid and the quasi-crystal graph for the hypoplactic monoid. Finally, the quasi-crystal structure is applied to prove some new results about the hypoplactic monoid.Comment: 55 pages. Minor revision to fix typos, add references, and discuss an open questio

Automaton semigroups: new construction results and examples of non-automaton semigroups

This paper studies the class of automaton semigroups from two perspectives: closure under constructions, and examples of semigroups that are not automaton semigroups. We prove that (semigroup) free products of finite semigroups always arise as automaton semigroups, and that the class of automaton monoids is closed under forming wreath products with finite monoids. We also consider closure under certain kinds of Rees matrix constructions, strong semilattices, and small extensions. Finally, we prove that no subsemigroup of $(\mathbb{N}, +)$ arises as an automaton semigroup. (Previously, $(\mathbb{N},+)$ itself was the unique example of a finitely generated residually finite semigroup that was known not to arise as an automaton semigroup.)Comment: 27 pages, 6 figures; substantially revise

Automaton semigroup constructions

The aim of this paper is to investigate whether the class of automaton semigroups is closed under certain semigroup constructions. We prove that the free product of two automaton semigroups that contain left identities is again an automaton semigroup. We also show that the class of automaton semigroups is closed under the combined operation of 'free product followed by adjoining an identity'. We present an example of a free product of finite semigroups that we conjecture is not an automaton semigroup. Turning to wreath products, we consider two slight generalizations of the concept of an automaton semigroup, and show that a wreath product of an automaton monoid and a finite monoid arises as a generalized automaton semigroup in both senses. We also suggest a potential counterexample that would show that a wreath product of an automaton monoid and a finite monoid is not a necessarily an automaton monoid in the usual sense.Comment: 13 pages; 2 figure

Hopfian and co-hopfian subsemigroups and extensions

This paper investigates the preservation of hopficity and co-hopficity on passing to finite-index subsemigroups and extensions. It was already known that hopficity is not preserved on passing to finite Rees index subsemigroups, even in the finitely generated case. We give a stronger example to show that it is not preserved even in the finitely presented case. It was also known that hopficity is not preserved in general on passing to finite Rees index extensions, but that it is preserved in the finitely generated case. We show that, in contrast, hopficity is not preserved on passing to finite Green index extensions, even within the class of finitely presented semigroups. Turning to co-hopficity, we prove that within the class of finitely generated semigroups, co-hopficity is preserved on passing to finite Rees index extensions, but is not preserved on passing to finite Rees index subsemigroups, even in the finitely presented case. Finally, by linking co-hopficity for graphs to co-hopficity for semigroups, we show that without the hypothesis of finite generation, co-hopficity is not preserved on passing to finite Rees index extensions.Comment: 15 pages; 3 figures. Revision to fix minor errors and add summary table