2,229 research outputs found
Optimal decision making for sperm chemotaxis in the presence of noise
For navigation, microscopic agents such as biological cells rely on noisy
sensory input. In cells performing chemotaxis, such noise arises from the
stochastic binding of signaling molecules at low concentrations. Using
chemotaxis of sperm cells as application example, we address the classic
problem of chemotaxis towards a single target. We reveal a fundamental
relationship between the speed of chemotactic steering and the strength of
directional fluctuations that result from the amplification of noise in the
chemical input signal. This relation implies a trade-off between slow, but
reliable, and fast, but less reliable, steering.
By formulating the problem of optimal navigation in the presence of noise as
a Markov decision process, we show that dynamic switching between reliable and
fast steering substantially increases the probability to find a target, such as
the egg. Intriguingly, this decision making would provide no benefit in the
absence of noise. Instead, decision making is most beneficial, if chemical
signals are above detection threshold, yet signal-to-noise ratios of gradient
measurements are low. This situation generically arises at intermediate
distances from a target, where signaling molecules emitted by the target are
diluted, thus defining a `noise zone' that cells have to cross.
Our work addresses the intermediate case between well-studied perfect
chemotaxis at high signal-to-noise ratios close to a target, and random search
strategies in the absence of navigation cues, e.g. far away from a target. Our
specific results provide a rational for the surprising observation of decision
making in recent experiments on sea urchin sperm chemotaxis. The general theory
demonstrates how decision making enables chemotactic agents to cope with high
levels of noise in gradient measurements by dynamically adjusting the
persistence length of a biased persistent random walk.Comment: 9 pages, 5 figure
Visualization and comparison of classical structures and quantum states of 4D maps
For generic 4D symplectic maps we propose the use of 3D phase-space slices
which allow for the global visualization of the geometrical organization and
coexistence of regular and chaotic motion. As an example we consider two
coupled standard maps. The advantages of the 3D phase-space slices are
presented in comparison to standard methods like 3D projections of orbits, the
frequency analysis, and a chaos indicator. Quantum mechanically, the 3D
phase-space slices allow for the first comparison of Husimi functions of
eigenstates of 4D maps with classical phase space structures. This confirms the
semi-classical eigenfunction hypothesis for 4D maps.Comment: For videos with rotated view of the 3D phase-space slices in high
resolution see http://www.comp-phys.tu-dresden.de/supp
Bifurcations of families of 1D-tori in 4D symplectic maps
The regular structures of a generic 4D symplectic map with a mixed phase
space are organized by one-parameter families of elliptic 1D-tori. Such
families show prominent bends, gaps, and new branches. We explain these
features in terms of bifurcations of the families when crossing a resonance.
For these bifurcations no external parameter has to be varied. Instead, the
longitudinal frequency, which varies along the family, plays the role of the
bifurcation parameter. As an example we study two coupled standard maps by
visualizing the elliptic and hyperbolic 1D-tori in a 3D phase-space slice,
local 2D projections, and frequency space. The observed bifurcations are
consistent with analytical predictions previously obtained for
quasi-periodically forced oscillators. Moreover, the new families emerging from
such a bifurcation form the skeleton of the corresponding resonance channel.Comment: 14 pages, 10 figures. For videos of 3D phase-space slices see
http://www.comp-phys.tu-dresden.de/supp
3D billiards: visualization of regular structures and trapping of chaotic trajectories
The dynamics in three-dimensional billiards leads, using a Poincar\'e
section, to a four-dimensional map which is challenging to visualize. By means
of the recently introduced 3D phase-space slices an intuitive representation of
the organization of the mixed phase space with regular and chaotic dynamics is
obtained. Of particular interest for applications are constraints to classical
transport between different regions of phase space which manifest in the
statistics of Poincar\'e recurrence times. For a 3D paraboloid billiard we
observe a slow power-law decay caused by long-trapped trajectories which we
analyze in phase space and in frequency space. Consistent with previous results
for 4D maps we find that: (i) Trapping takes place close to regular structures
outside the Arnold web. (ii) Trapping is not due to a generalized
island-around-island hierarchy. (iii) The dynamics of sticky orbits is governed
by resonance channels which extend far into the chaotic sea. We find clear
signatures of partial transport barriers. Moreover, we visualize the geometry
of stochastic layers in resonance channels explored by sticky orbits.Comment: 20 pages, 11 figures. For videos of 3D phase-space slices and
time-resolved animations see http://www.comp-phys.tu-dresden.de/supp
The eDAL Suite: Tools and Concepts for Primary Data Citation
Retrieval and citation of primary data is the important factor in the approaching age of “data science”. Digital data are easily shared, and just as easily wiped or lost. The problem of keeping on-line data accessible and
retrievable is especially difficult for SME like plant breeders plant biotech companies as well as research projects in this domain.
Intension of eDAL is the provisioning of an information retrieval and data citation infrastructure that meets the requirements of the “data science” age and implements a re-usable platform for data retrieval, data
citation, and data publication. Like a shopping cart, the idea is to combine a search engine and a data cart, which retrieves, rank and collect query relevant data from crop plant data centers
Android Security, Pitfalls, Lessons Learned and BYOD
Over the last two years Android became the most popular mobile operating system. But Android is also targeted by an over-proportional share of malware. In this paper we systematize the knowledge about the Android security mechanisms and formulate how the pitfalls can be avoided when building a mobile operating system. As smartphones enter the corporate domain, a new scheme called bring your own device (BYOD) became popular. One solution is to logically partition the device such that personal and business information are isolated from one another. We systematize the solutions for partitioning in Android
Surfing along concentration filaments: sperm chemotaxis in physiological shear flows
Many motile biological cells navigate along concentration gradients of
signaling molecules: This chemotaxis guides for instance sperm cells from
marine invertebrates, which have to find egg cells in the ocean. While
chemotaxis has been intensively studied for idealized conditions of
rotationally symmetric gradients in still water, natural gradients are usually
distorted, e.g., by turbulent flows in the ocean. Recent experiments and direct
numerical simulations with sperm cells and bacteria surprisingly suggest the
existence of an optimal flow strength at which chemotaxis is more effective
than for still water. We use sperm chemotaxis in simple shear flow as a
prototypical example to understand the origin of such an optimal flow strength
theoretically: We quantify how flow accelerates spreading of signaling
molecules released by the egg, but distorts the resulting concentration field
into long and thin filaments. The competition between these two effects sets an
optimal flow strength that maximizes sperm-egg encounter. We characterize how
sperm cells `surf' along concentration filaments, typical for scalar
turbulence, revealing a general navigation paradigm in the presence of flow. We
compare both simulation and theory with previous experimental results and find
good agreement.Comment: manuscript: 6 pages, 4 figures; SI: 11 pages, 8 figure
Globaler SĂźden, Kapitalismus und die Zukunft von Degrowth
GroĂe Fragen wie âPostwachstum fĂźr den Globalen SĂźden?â und âDegrowth im Kapitalismus?â standen im Mittelpunkt der fĂźnften Degrowth-Konferenz. Neben den inhaltlichen Diskussionen ist nun auch eine Debatte Ăźber die Zukunft der Degrowth- Bewegung entbrannt
Rahmenbedingungen einer PostwachstumsĂśkonomie
Eine Wirtschaft jenseits des Wachstums bedarf grundlegender makroÜkonomischer Verände rungen. Sie betreffen das Steuersystem, die Unternehmensformen, die Verteilung des Wohlstands und die Arbeitszeiten. Doch auch Werbung und Bildung mßssen sich wandeln
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