780 research outputs found
Linear-Time FPT Algorithms via Network Flow
In the area of parameterized complexity, to cope with NP-Hard problems, we
introduce a parameter k besides the input size n, and we aim to design
algorithms (called FPT algorithms) that run in O(f(k)n^d) time for some
function f(k) and constant d. Though FPT algorithms have been successfully
designed for many problems, typically they are not sufficiently fast because of
huge f(k) and d. In this paper, we give FPT algorithms with small f(k) and d
for many important problems including Odd Cycle Transversal and Almost 2-SAT.
More specifically, we can choose f(k) as a single exponential (4^k) and d as
one, that is, linear in the input size. To the best of our knowledge, our
algorithms achieve linear time complexity for the first time for these
problems. To obtain our algorithms for these problems, we consider a large
class of integer programs, called BIP2. Then we show that, in linear time, we
can reduce BIP2 to Vertex Cover Above LP preserving the parameter k, and we can
compute an optimal LP solution for Vertex Cover Above LP using network flow.
Then, we perform an exhaustive search by fixing half-integral values in the
optimal LP solution for Vertex Cover Above LP. A bottleneck here is that we
need to recompute an LP optimal solution after branching. To address this
issue, we exploit network flow to update the optimal LP solution in linear
time.Comment: 20 page
Half-integrality, LP-branching and FPT Algorithms
A recent trend in parameterized algorithms is the application of polytope
tools (specifically, LP-branching) to FPT algorithms (e.g., Cygan et al., 2011;
Narayanaswamy et al., 2012). However, although interesting results have been
achieved, the methods require the underlying polytope to have very restrictive
properties (half-integrality and persistence), which are known only for few
problems (essentially Vertex Cover (Nemhauser and Trotter, 1975) and Node
Multiway Cut (Garg et al., 1994)). Taking a slightly different approach, we
view half-integrality as a \emph{discrete} relaxation of a problem, e.g., a
relaxation of the search space from to such that
the new problem admits a polynomial-time exact solution. Using tools from CSP
(in particular Thapper and \v{Z}ivn\'y, 2012) to study the existence of such
relaxations, we provide a much broader class of half-integral polytopes with
the required properties, unifying and extending previously known cases.
In addition to the insight into problems with half-integral relaxations, our
results yield a range of new and improved FPT algorithms, including an
-time algorithm for node-deletion Unique Label Cover with
label set and an -time algorithm for Group Feedback Vertex
Set, including the setting where the group is only given by oracle access. All
these significantly improve on previous results. The latter result also implies
the first single-exponential time FPT algorithm for Subset Feedback Vertex Set,
answering an open question of Cygan et al. (2012).
Additionally, we propose a network flow-based approach to solve some cases of
the relaxation problem. This gives the first linear-time FPT algorithm to
edge-deletion Unique Label Cover.Comment: Added results on linear-time FPT algorithms (not present in SODA
paper
Chemical potential jump between hole- and electron-doped sides of ambipolar high-Tc cuprate
In order to study an intrinsic chemical potential jump between the hole- and
electron-doped high-Tc superconductors, we have performed core-level X-ray
photoemission spectroscopy (XPS) measurements of Y0.38La0.62Ba1.74La0.26Cu3Oy
(YLBLCO), into which one can dope both holes and electrons with maintaining the
same crystal structure. Unlike the case between the hole-doped system
La_2-xSrxCuO4 and the electron-doped system Nd_2-xCexCuO4, we have estimated
the true chemical potential jump between the hole- and electron-doped YLBLCO to
be ~0.8 eV, which is much smaller than the optical gaps of 1.4-1.7 eV reported
for the parent insulating compounds. We attribute the reduced jump to the
indirect nature of the charge-excitation gap as well as to the polaronic nature
of the doped carriers.Comment: 4 pages, 3 figure
Fast Exact Shortest-Path Distance Queries on Large Networks by Pruned Landmark Labeling
We propose a new exact method for shortest-path distance queries on
large-scale networks. Our method precomputes distance labels for vertices by
performing a breadth-first search from every vertex. Seemingly too obvious and
too inefficient at first glance, the key ingredient introduced here is pruning
during breadth-first searches. While we can still answer the correct distance
for any pair of vertices from the labels, it surprisingly reduces the search
space and sizes of labels. Moreover, we show that we can perform 32 or 64
breadth-first searches simultaneously exploiting bitwise operations. We
experimentally demonstrate that the combination of these two techniques is
efficient and robust on various kinds of large-scale real-world networks. In
particular, our method can handle social networks and web graphs with hundreds
of millions of edges, which are two orders of magnitude larger than the limits
of previous exact methods, with comparable query time to those of previous
methods.Comment: To appear in SIGMOD 201
Substellar Companions to Seven Evolved Intermediate-Mass Stars
We report the detections of substellar companions orbiting around seven
evolved intermediate-mass stars from precise Doppler measurements at Okayama
Astrophysical Observatory. o UMa (G4 II-III) is a giant with a mass of 3.1
M_sun and hosts a planet with minimum mass of m_2sini=4.1 M_J in an orbit with
a period P=1630 d and an eccentricity e=0.13. This is the first planet
candidate (< 13 M_J) ever discovered around stars more massive than 3 M_sun. o
CrB (K0 III) is a 2.1 M_sun giant and has a planet of m_2sini=1.5 M_J in a
187.8 d orbit with e=0.19. This is one of the least massive planets ever
discovered around ~2 M_sun stars. HD 5608 (K0 IV) is an 1.6 M_sun subgiant
hosting a planet of m_2sini=1.4 M_J in a 793 d orbit with e=0.19. The star also
exhibits a linear velocity trend suggesting the existence of an outer, more
massive companion. 75 Cet (G3 III:) is a 2.5 M_sun giant hosting a planet of
m_2sini=3.0 M_J in a 692 d orbit with e=0.12. The star also shows possible
additional periodicity of about 200 d and 1880 d with velocity amplitude of
~7--10 m/s, although these are not significant at this stage. nu Oph (K0 III)
is a 3.0 M_sun giant and has two brown-dwarf companions of m_2sini= 24 M_J and
27 M_J, in orbits with P=530.3 d and 3190 d, and e=0.126 and 0.17,
respectively, which were independently announced by Quirrenbach et al. (2011).
The ratio of the periods is close to 1:6, suggesting that the companions are in
mean motion resonance. We also independently confirmed planets around k CrB (K0
III-IV) and HD 210702 (K1 IV), which had been announced by Johnson et al.
(2008) and Johnson et al. (2007a), respectively. All of the orbital parameters
we obtained are consistent with the previous results.Comment: 21 pages, 14 figures, accepted for publication in PAS
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