4,235 research outputs found
On the Order Type of Scattered Context-Free Orderings
We show that if a context-free grammar generates a language whose
lexicographic ordering is well-ordered of type less than , then its
order type is effectively computable.Comment: In Proceedings GandALF 2019, arXiv:1909.05979. arXiv admin note: text
overlap with arXiv:1907.1157
The FC-rank of a context-free language
We prove that the finite condensation rank (FC-rank) of the lexicographic
ordering of a context-free language is strictly less than
The Rank of Tree-Automatic Linear Orderings
We generalise Delhomm\'e's result that each tree-automatic ordinal is
strictly below \omega^\omega^\omega{} by showing that any tree-automatic linear
ordering has FC-rank strictly below \omega^\omega. We further investigate a
restricted form of tree-automaticity and prove that every linear ordering which
admits a tree-automatic presentation of branching complexity at most k has
FC-rank strictly below \omega^k.Comment: 20 pages, 3 figure
On factorisation forests
The theorem of factorisation forests shows the existence of nested
factorisations -- a la Ramsey -- for finite words. This theorem has important
applications in semigroup theory, and beyond. The purpose of this paper is to
illustrate the importance of this approach in the context of automata over
infinite words and trees. We extend the theorem of factorisation forest in two
directions: we show that it is still valid for any word indexed by a linear
ordering; and we show that it admits a deterministic variant for words indexed
by well-orderings. A byproduct of this work is also an improvement on the known
bounds for the original result. We apply the first variant for giving a
simplified proof of the closure under complementation of rational sets of words
indexed by countable scattered linear orderings. We apply the second variant in
the analysis of monadic second-order logic over trees, yielding new results on
monadic interpretations over trees. Consequences of it are new caracterisations
of prefix-recognizable structures and of the Caucal hierarchy.Comment: 27 page
Laver and set theory
In this commemorative article, the work of Richard Laver is surveyed in its full range and extent.Accepted manuscrip
Event reconstruction in high resolution Compton telescopes
The development of germanium Compton telescopes for nuclear gamma-ray
astrophysics (~0.2-20 MeV) requires new event reconstruction techniques to
accurately determine the initial direction and energy of photon events, as well
as to consistently reject background events. This paper describes techniques
for event reconstruction, accounting for realistic instrument/detector
performance and uncertainties. An especially important technique is Compton
Kinematic Discrimination, which allows proper interaction ordering and
background rejection with high probabilities. The use of these techniques are
crucial for the realistic evaluation of the performance and sensitivity of any
germanium Compton telescope configuration.Comment: Accepted for publication in A&AS
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