9,239 research outputs found
Ocean odours
The ocean's distinctive smell is caused by a single chemical released by plankton and other marine life, dimethyl sulphide (DMS). A study by a group of investigators from the University of Groningen used a technique called laser-sheet particle image velocimetry to monitor the water flows produced by aquatic animals. The investigators looked closely at how DMS affects copepods. Their tests showed that when DMS hit a copepod, the test animal reacted with a search behaviour. This demonstrates that copepods can smell the DMS and suggests that this and possibly other compounds released by phytoplankton and microzooplankton may help copepods in finding their prey
Effective String Theory of Vortices and Regge Trajectories of Hybrid Mesons with Zero Mass Quarks
We show how a field theory containing classical vortex solutions can be
expressed as an effective string theory of long distance QCD describing the two
transverse oscillations of the string. We use the semiclassical expansion of
this effective string theory about a classical rotating string solution to
obtain Regge trajectories for mesons with zero mass quarks. The first
semiclassical correction adds the constant 1/12 to the classical Regge formula
for the angular momentum of mesons on the leading Regge trajectory. In D
spacetime dimensions, this additive constant is (D-2)/24. The excited states of
the rotating string give rise to daughter Regge trajectories determining the
spectrum of hybrid mesons.Comment: 12 pages, 2 figures, LaTeX, style file include
Weighted Polynomial Approximations: Limits for Learning and Pseudorandomness
Polynomial approximations to boolean functions have led to many positive
results in computer science. In particular, polynomial approximations to the
sign function underly algorithms for agnostically learning halfspaces, as well
as pseudorandom generators for halfspaces. In this work, we investigate the
limits of these techniques by proving inapproximability results for the sign
function.
Firstly, the polynomial regression algorithm of Kalai et al. (SIAM J. Comput.
2008) shows that halfspaces can be learned with respect to log-concave
distributions on in the challenging agnostic learning model. The
power of this algorithm relies on the fact that under log-concave
distributions, halfspaces can be approximated arbitrarily well by low-degree
polynomials. We ask whether this technique can be extended beyond log-concave
distributions, and establish a negative result. We show that polynomials of any
degree cannot approximate the sign function to within arbitrarily low error for
a large class of non-log-concave distributions on the real line, including
those with densities proportional to .
Secondly, we investigate the derandomization of Chernoff-type concentration
inequalities. Chernoff-type tail bounds on sums of independent random variables
have pervasive applications in theoretical computer science. Schmidt et al.
(SIAM J. Discrete Math. 1995) showed that these inequalities can be established
for sums of random variables with only -wise independence,
for a tail probability of . We show that their results are tight up to
constant factors.
These results rely on techniques from weighted approximation theory, which
studies how well functions on the real line can be approximated by polynomials
under various distributions. We believe that these techniques will have further
applications in other areas of computer science.Comment: 22 page
Tight Lower Bounds for Differentially Private Selection
A pervasive task in the differential privacy literature is to select the
items of "highest quality" out of a set of items, where the quality of each
item depends on a sensitive dataset that must be protected. Variants of this
task arise naturally in fundamental problems like feature selection and
hypothesis testing, and also as subroutines for many sophisticated
differentially private algorithms.
The standard approaches to these tasks---repeated use of the exponential
mechanism or the sparse vector technique---approximately solve this problem
given a dataset of samples. We provide a tight lower
bound for some very simple variants of the private selection problem. Our lower
bound shows that a sample of size is required
even to achieve a very minimal accuracy guarantee.
Our results are based on an extension of the fingerprinting method to sparse
selection problems. Previously, the fingerprinting method has been used to
provide tight lower bounds for answering an entire set of queries, but
often only some much smaller set of queries are relevant. Our extension
allows us to prove lower bounds that depend on both the number of relevant
queries and the total number of queries
Modeling Time-dependent CO Intensities in Multi-modal Energy Systems with Storage
CO emission reduction and increasing volatile renewable energy generation
mandate stronger energy sector coupling and the use of energy storage. In such
multi-modal energy systems, it is challenging to determine the effect of an
individual player's consumption pattern onto overall CO emissions. This,
however, is often important to evaluate the suitability of local CO
reduction measures. Due to renewables' volatility, the traditional approach of
using annual average CO intensities per energy form is no longer accurate,
but the time of consumption should be considered. Moreover, CO intensities
are highly coupled over time and different energy forms due to sector coupling
and energy storage. We introduce and compare two novel methods for computing
time-dependent CO intensities, that address different objectives: the first
method determines CO intensities of the energy system as is. The second
method analyzes how overall CO emissions would change in response to
infinitesimal demand changes. Given a digital twin of the energy system in form
of a linear program, we show how to compute these sensitivities very
efficiently. We present the results of both methods for two simulated test
energy systems and discuss their different implications.Comment: This work has been submitted to the Elsevier Applied Energy for
possible publication. Copyright may be transferred without notice, after
which this version may no longer be accessibl
Application of a Two-dimensional Unsteady Viscous Analysis Code to a Supersonic Throughflow Fan Stage
The Rai ROTOR1 code for two-dimensional, unsteady viscous flow analysis was applied to a supersonic throughflow fan stage design. The axial Mach number for this fan design increases from 2.0 at the inlet to 2.9 at the outlet. The Rai code uses overlapped O- and H-grids that are appropriately packed. The Rai code was run on a Cray XMP computer; then data postprocessing and graphics were performed to obtain detailed insight into the stage flow. The large rotor wakes uniformly traversed the rotor-stator interface and dispersed as they passed through the stator passage. Only weak blade shock losses were computerd, which supports the design goals. High viscous effects caused large blade wakes and a low fan efficiency. Rai code flow predictions were essentially steady for the rotor, and they compared well with Chima rotor viscous code predictions based on a C-grid of similar density
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