292 research outputs found
Shortest paths between shortest paths and independent sets
We study problems of reconfiguration of shortest paths in graphs. We prove
that the shortest reconfiguration sequence can be exponential in the size of
the graph and that it is NP-hard to compute the shortest reconfiguration
sequence even when we know that the sequence has polynomial length. Moreover,
we also study reconfiguration of independent sets in three different models and
analyze relationships between these models, observing that shortest path
reconfiguration is a special case of independent set reconfiguration in perfect
graphs, under any of the three models. Finally, we give polynomial results for
restricted classes of graphs (even-hole-free and -free graphs)
Gravity waves and high-altitude CO ice cloud formation in the Martian atmosphere
We present the first general circulation model simulations that quantify and
reproduce patches of extremely cold air required for CO condensation and
cloud formation in the Martian mesosphere. They are created by subgrid-scale
gravity waves (GWs) accounted for in the model with the interactively
implemented spectral parameterization. Distributions of GW-induced temperature
fluctuations and occurrences of supersaturation conditions are in a good
agreement with observations of high-altitude CO ice clouds. Our study
confirms the key role of GWs in facilitating CO cloud formation, discusses
their tidal modulation, and predicts clouds at altitudes higher than have been
observed to date.Comment: Accepted for publication in Geophysical Research Letters (GRL
Theoretical analysis of edit distance algorithms: an applied perspective
Given its status as a classic problem and its importance to both
theoreticians and practitioners, edit distance provides an excellent lens
through which to understand how the theoretical analysis of algorithms impacts
practical implementations. From an applied perspective, the goals of
theoretical analysis are to predict the empirical performance of an algorithm
and to serve as a yardstick to design novel algorithms that perform well in
practice. In this paper, we systematically survey the types of theoretical
analysis techniques that have been applied to edit distance and evaluate the
extent to which each one has achieved these two goals. These techniques include
traditional worst-case analysis, worst-case analysis parametrized by edit
distance or entropy or compressibility, average-case analysis, semi-random
models, and advice-based models. We find that the track record is mixed. On one
hand, two algorithms widely used in practice have been born out of theoretical
analysis and their empirical performance is captured well by theoretical
predictions. On the other hand, all the algorithms developed using theoretical
analysis as a yardstick since then have not had any practical relevance. We
conclude by discussing the remaining open problems and how they can be tackled
Informed and Automated k-Mer Size Selection for Genome Assembly
Genome assembly tools based on the de Bruijn graph framework rely on a
parameter k, which represents a trade-off between several competing effects
that are difficult to quantify. There is currently a lack of tools that would
automatically estimate the best k to use and/or quickly generate histograms of
k-mer abundances that would allow the user to make an informed decision.
We develop a fast and accurate sampling method that constructs approximate
abundance histograms with a several orders of magnitude performance improvement
over traditional methods. We then present a fast heuristic that uses the
generated abundance histograms for putative k values to estimate the best
possible value of k. We test the effectiveness of our tool using diverse
sequencing datasets and find that its choice of k leads to some of the best
assemblies.
Our tool KmerGenie is freely available at: http://kmergenie.bx.psu.edu/Comment: HiTSeq 201
Seasonal Water "Pump" in the Atmosphere of Mars: Vertical Transport to the Thermosphere
We present results of simulations with the Max Planck Institute general
circulation model (MPI-MGCM) implementing a hydrological cycle scheme. The
simulations reveal a seasonal water "pump" mechanism responsible for the upward
transport of water vapor. This mechanism occurs in high latitudes above
60 of the southern hemisphere at perihelion, when the upward branch of
the meridional circulation is particularly strong. A combination of the mean
vertical flux with variations induced by solar tides facilitates penetration of
water across the "bottleneck" at approximately 60 km. The meridional
circulation then transports water across the globe to the northern hemisphere.
Since the intensity of the meridional cell is tightly controlled by airborne
dust, the water abundance in the thermosphere strongly increases during dust
storms.Comment: 15 pages, 4 figure
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