3,283 research outputs found
Geometric realizations of two dimensional substitutive tilings
We define 2-dimensional topological substitutions. A tiling of the Euclidean
plane, or of the hyperbolic plane, is substitutive if the underlying 2-complex
can be obtained by iteration of a 2-dimensional topological substitution. We
prove that there is no primitive substitutive tiling of the hyperbolic plane
. However, we give an example of substitutive tiling of \Hyp^2
which is non-primitive.Comment: 30 pages, 13 figure
On Quasi-Interpretations, Blind Abstractions and Implicit Complexity
Quasi-interpretations are a technique to guarantee complexity bounds on
first-order functional programs: with termination orderings they give in
particular a sufficient condition for a program to be executable in polynomial
time, called here the P-criterion. We study properties of the programs
satisfying the P-criterion, in order to better understand its intensional
expressive power. Given a program on binary lists, its blind abstraction is the
nondeterministic program obtained by replacing lists by their lengths (natural
numbers). A program is blindly polynomial if its blind abstraction terminates
in polynomial time. We show that all programs satisfying a variant of the
P-criterion are in fact blindly polynomial. Then we give two extensions of the
P-criterion: one by relaxing the termination ordering condition, and the other
one (the bounded value property) giving a necessary and sufficient condition
for a program to be polynomial time executable, with memoisation.Comment: 18 page
Extremal words in morphic subshifts
Given an infinite word X over an alphabet A a letter b occurring in X, and a
total order \sigma on A, we call the smallest word with respect to \sigma
starting with b in the shift orbit closure of X an extremal word of X. In this
paper we consider the extremal words of morphic words. If X = g(f^{\omega}(a))
for some morphisms f and g, we give two simple conditions on f and g that
guarantees that all extremal words are morphic. This happens, in particular,
when X is a primitive morphic or a binary pure morphic word. Our techniques
provide characterizations of the extremal words of the Period-doubling word and
the Chacon word and give a new proof of the form of the lexicographically least
word in the shift orbit closure of the Rudin-Shapiro word.Comment: Replaces a previous version entitled "Extremal words in the shift
orbit closure of a morphic sequence" with an added result on primitive
morphic sequences. Submitte
Signals of demographic expansion in Drosophila virilis
BACKGROUND:
The pattern of genetic variation within and among populations of a species is strongly affected by its phylogeographic history. Analyses based on putatively neutral markers provide data from which past events, such as population expansions and colonizations, can be inferred. Drosophila virilis is a cosmopolitan species belonging to the virilis group, where divergence times between different phylads go back to the early Miocene. We analysed mitochondrial DNA sequence variation among 35 Drosophila virilis strains covering the species' range in order to detect demographic events that could be used to understand the present characteristics of the species, as well as its differences from other members of the group.
RESULTS:
Drosophila virilis showed very low nucleotide diversity with haplotypes distributed in a star-like network, consistent with a recent world-wide exponential expansion possibly associated either with domestication or post-glacial colonization. All analyses point towards a rapid population expansion. Coalescence models support this interpretation. The central haplotype in the network, which could be interpreted as ancestral, is widely distributed and gives no information about the geographical origin of the population expansion. The species showed no geographic structure in the distribution of mitochondrial haplotypes, in contrast to results of a recent microsatellite-based analysis.
CONCLUSION:
The lack of geographic structure and the star-like topology depicted by the D. virilis haplotypes indicate a pattern of global demographic expansion, probably related to human movements, although this interpretation cannot be distinguished from a selective sweep in the mitochondrial DNA until nuclear sequence data become available. The particular behavioural traits of this species, including weak species-discrimination and intraspecific mate choice exercised by the females, can be understood from this perspective
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