50,127 research outputs found
Regular Cost Functions, Part I: Logic and Algebra over Words
The theory of regular cost functions is a quantitative extension to the
classical notion of regularity. A cost function associates to each input a
non-negative integer value (or infinity), as opposed to languages which only
associate to each input the two values "inside" and "outside". This theory is a
continuation of the works on distance automata and similar models. These models
of automata have been successfully used for solving the star-height problem,
the finite power property, the finite substitution problem, the relative
inclusion star-height problem and the boundedness problem for monadic-second
order logic over words. Our notion of regularity can be -- as in the classical
theory of regular languages -- equivalently defined in terms of automata,
expressions, algebraic recognisability, and by a variant of the monadic
second-order logic. These equivalences are strict extensions of the
corresponding classical results. The present paper introduces the cost monadic
logic, the quantitative extension to the notion of monadic second-order logic
we use, and show that some problems of existence of bounds are decidable for
this logic. This is achieved by introducing the corresponding algebraic
formalism: stabilisation monoids.Comment: 47 page
Gamma-Set Domination Graphs. I: Complete Biorientations of \u3cem\u3eq-\u3c/em\u3eExtended Stars and Wounded Spider Graphs
The domination number of a graph G, γ(G), and the domination graph of a digraph D, dom(D) are integrated in this paper. The γ-set domination graph of the complete biorientation of a graph G, domγ(G) is created. All γ-sets of specific trees T are found, and dom-γ(T) is characterized for those classes
Separating regular languages with two quantifier alternations
We investigate a famous decision problem in automata theory: separation.
Given a class of language C, the separation problem for C takes as input two
regular languages and asks whether there exists a third one which belongs to C,
includes the first one and is disjoint from the second. Typically, obtaining an
algorithm for separation yields a deep understanding of the investigated class
C. This explains why a lot of effort has been devoted to finding algorithms for
the most prominent classes.
Here, we are interested in classes within concatenation hierarchies. Such
hierarchies are built using a generic construction process: one starts from an
initial class called the basis and builds new levels by applying generic
operations. The most famous one, the dot-depth hierarchy of Brzozowski and
Cohen, classifies the languages definable in first-order logic. Moreover, it
was shown by Thomas that it corresponds to the quantifier alternation hierarchy
of first-order logic: each level in the dot-depth corresponds to the languages
that can be defined with a prescribed number of quantifier blocks. Finding
separation algorithms for all levels in this hierarchy is among the most famous
open problems in automata theory.
Our main theorem is generic: we show that separation is decidable for the
level 3/2 of any concatenation hierarchy whose basis is finite. Furthermore, in
the special case of the dot-depth, we push this result to the level 5/2. In
logical terms, this solves separation for : first-order sentences
having at most three quantifier blocks starting with an existential one
Potential-controlled filtering in quantum star graphs
We study the scattering in a quantum star graph with a F\"ul\"op--Tsutsui
coupling in its vertex and with external potentials on the lines. We find
certain special couplings for which the probability of the transmission between
two given lines of the graph is strongly influenced by the potential applied on
another line. On the basis of this phenomenon we design a tunable quantum
band-pass spectral filter. The transmission from the input to the output line
is governed by a potential added on the controlling line. The strength of the
potential directly determines the passband position, which allows to control
the filter in a macroscopic manner. Generalization of this concept to quantum
devices with multiple controlling lines proves possible. It enables the
construction of spectral filters with more controllable parameters or with more
operation modes. In particular, we design a band-pass filter with independently
adjustable multiple passbands. We also address the problem of the physical
realization of F\"ul\"op--Tsutsui couplings and demonstrate that the couplings
needed for the construction of the proposed quantum devices can be approximated
by simple graphs carrying only potentials.Comment: 41 pages, 17 figure
Modified mean curvature flow of star-shaped hypersurfaces in hyperbolic space
We define a new version of modified mean curvature flow (MMCF) in hyperbolic
space , which interestingly turns out to be the natural
negative -gradient flow of the energy functional defined by De Silva and
Spruck in \cite{DS09}. We show the existence, uniqueness and convergence of the
MMCF of complete embedded star-shaped hypersurfaces with fixed prescribed
asymptotic boundary at infinity. As an application, we recover the existence
and uniqueness of smooth complete hypersurfaces of constant mean curvature in
hyperbolic space with prescribed asymptotic boundary at infinity, which was
first shown by Guan and Spruck.Comment: 26 pages, 3 figure
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