4,381 research outputs found
Forbidden induced subgraphs and the price of connectivity for feedback vertex set.
Let fvs(G) and cfvs(G) denote the cardinalities of a minimum feedback vertex set and a minimum connected feedback vertex set of a graph G, respectively. For a graph class G, the price of connectivity for feedback vertex set (poc-fvs) for G is defined as the maximum ratio cfvs(G)/fvs(G) over all connected graphs G in G. It is known that the poc-fvs for general graphs is unbounded. We study the poc-fvs for graph classes defined by a finite family H of forbidden induced subgraphs. We characterize exactly those finite families H for which the poc-fvs for H-free graphs is bounded by a constant. Prior to our work, such a result was only known for the case where |H|=1
A reconfigurations analogue of Brooks’ theorem.
Let G be a simple undirected graph on n vertices with maximum degree Δ. Brooks’ Theorem states that G has a Δ-colouring unless G is a complete graph, or a cycle with an odd number of vertices. To recolour G is to obtain a new proper colouring by changing the colour of one vertex. We show that from a k-colouring, k > Δ, a Δ-colouring of G can be obtained by a sequence of O(n 2) recolourings using only the original k colours unless
G is a complete graph or a cycle with an odd number of vertices, or
k = Δ + 1, G is Δ-regular and, for each vertex v in G, no two neighbours of v are coloured alike.
We use this result to study the reconfiguration graph R k (G) of the k-colourings of G. The vertex set of R k (G) is the set of all possible k-colourings of G and two colourings are adjacent if they differ on exactly one vertex. It is known that
if k ≤ Δ(G), then R k (G) might not be connected and it is possible that its connected components have superpolynomial diameter,
if k ≥ Δ(G) + 2, then R k (G) is connected and has diameter O(n 2).
We complete this structural classification by settling the missing case:
if k = Δ(G) + 1, then R k (G) consists of isolated vertices and at most one further component which has diameter O(n 2).
We also describe completely the computational complexity classification of the problem of deciding whether two k-colourings of a graph G of maximum degree Δ belong to the same component of R k (G) by settling the case k = Δ(G) + 1. The problem is
O(n 2) time solvable for k = 3,
PSPACE-complete for 4 ≤ k ≤ Δ(G),
O(n) time solvable for k = Δ(G) + 1,
O(1) time solvable for k ≥ Δ(G) + 2 (the answer is always yes)
On Pebble Automata for Data Languages with Decidable Emptiness Problem
In this paper we study a subclass of pebble automata (PA) for data languages
for which the emptiness problem is decidable. Namely, we introduce the
so-called top view weak PA. Roughly speaking, top view weak PA are weak PA
where the equality test is performed only between the data values seen by the
two most recently placed pebbles. The emptiness problem for this model is
decidable. We also show that it is robust: alternating, nondeterministic and
deterministic top view weak PA have the same recognition power. Moreover, this
model is strong enough to accept all data languages expressible in Linear
Temporal Logic with the future-time operators, augmented with one register
freeze quantifier.Comment: An extended abstract of this work has been published in the
proceedings of the 34th International Symposium on Mathematical Foundations
of Computer Science (MFCS) 2009}, Springer, Lecture Notes in Computer Science
5734, pages 712-72
The tractability frontier of well-designed SPARQL queries
We study the complexity of query evaluation of SPARQL queries. We focus on
the fundamental fragment of well-designed SPARQL restricted to the AND,
OPTIONAL and UNION operators. Our main result is a structural characterisation
of the classes of well-designed queries that can be evaluated in polynomial
time. In particular, we introduce a new notion of width called domination
width, which relies on the well-known notion of treewidth. We show that, under
some complexity theoretic assumptions, the classes of well-designed queries
that can be evaluated in polynomial time are precisely those of bounded
domination width
An automaton over data words that captures EMSO logic
We develop a general framework for the specification and implementation of
systems whose executions are words, or partial orders, over an infinite
alphabet. As a model of an implementation, we introduce class register
automata, a one-way automata model over words with multiple data values. Our
model combines register automata and class memory automata. It has natural
interpretations. In particular, it captures communicating automata with an
unbounded number of processes, whose semantics can be described as a set of
(dynamic) message sequence charts. On the specification side, we provide a
local existential monadic second-order logic that does not impose any
restriction on the number of variables. We study the realizability problem and
show that every formula from that logic can be effectively, and in elementary
time, translated into an equivalent class register automaton
Absolute properties of the main-sequence eclipsing binary FM Leo
First spectroscopic and new photometric observations of the eclipsing binary
FM Leo are presented. The main aims were to determine orbital and stellar
parameters of two components and their evolutionary stage. First spectroscopic
observations of the system were obtained with DDO and PST spectrographs. The
results of the orbital solution from radial velocity curves are combined with
those derived from the light-curve analysis (ASAS-3 photometry and
supplementary observations of eclipses with 1 m and 0.35 m telescopes) to
derive orbital and stellar parameters. JKTEBOP, Wilson-Devinney binary
modelling codes and a two-dimensional cross-correlation (TODCOR) method were
applied for the analysis. We find the masses to be M_1 = 1.318 0.007 and
M_2 = 1.287 0.007 M_sun, the radii to be R_1 = 1.648 0.043 and R_2
= 1.511 0.049 R_sun for primary and secondary stars, respectively. The
evolutionary stage of the system is briefly discussed by comparing physical
parameters with current stellar evolution models. We find the components are
located at the main sequence, with an age of about 3 Gyr.Comment: 5 pages, 4 figures, to appear in MNRA
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