64 research outputs found
Deterministic Symmetry Breaking in Ring Networks
We study a distributed coordination mechanism for uniform agents located on a
circle. The agents perform their actions in synchronised rounds. At the
beginning of each round an agent chooses the direction of its movement from
clockwise, anticlockwise, or idle, and moves at unit speed during this round.
Agents are not allowed to overpass, i.e., when an agent collides with another
it instantly starts moving with the same speed in the opposite direction
(without exchanging any information with the other agent). However, at the end
of each round each agent has access to limited information regarding its
trajectory of movement during this round.
We assume that mobile agents are initially located on a circle unit
circumference at arbitrary but distinct positions unknown to other agents. The
agents are equipped with unique identifiers from a fixed range. The {\em
location discovery} task to be performed by each agent is to determine the
initial position of every other agent.
Our main result states that, if the only available information about movement
in a round is limited to %information about distance between the initial and
the final position, then there is a superlinear lower bound on time needed to
solve the location discovery problem. Interestingly, this result corresponds to
a combinatorial symmetry breaking problem, which might be of independent
interest. If, on the other hand, an agent has access to the distance to its
first collision with another agent in a round, we design an asymptotically
efficient and close to optimal solution for the location discovery problem.Comment: Conference version accepted to ICDCS 201
On the Impact of Geometry on Ad Hoc Communication in Wireless Networks
In this work we address the question how important is the knowledge of
geometric location and network density to the efficiency of (distributed)
wireless communication in ad hoc networks. We study fundamental communication
task of broadcast and develop well-scalable, randomized algorithms that do not
rely on GPS information, and which efficiency formulas do not depend on how
dense the geometric network is. We consider two settings: with and without
spontaneous wake-up of nodes. In the former setting, in which all nodes start
the protocol at the same time, our algorithm accomplishes broadcast in rounds under the SINR model, with high probability (whp), where
is the diameter of the communication graph and is the number of
stations. In the latter setting, in which only the source node containing the
original message is active in the beginning, we develop a slightly slower
algorithm working in rounds whp. Both algorithms are based on a
novel distributed coloring method, which is of independent interest and
potential applicability to other communication tasks under the SINR wireless
model
Proofs of Knowledge with Several Challenge Values
In this paper we consider the problem of increasing
the number of possible challenge values from 2 to
in various zero-knowledge cut and choose protocols.
First we discuss doing this for graph isomorphism protocol.
Then we show how increasing this number improves efficiency
of protocols for double discrete logarithm
and -th root of discrete logarithm which are potentially very useful
tools for constructing complex cryptographic protocols.
The practical improvement given by our paper is 2-4 times in terms
of both time complexity and transcript size
Brief Announcement: New Clocks, Fast Line Formation and Self-Replication Population Protocols
In this paper we consider a known variant of the standard population protocol model in which agents can be connected by edges, referred to as the network constructor model. During an interaction between two agents the relevant connecting edge can be formed, maintained or eliminated by the transition function. The state space of agents is fixed (constant size) and the size n of the population is not known, i.e., not hard-coded in the transition function. Since pairs of agents are chosen uniformly at random the status of each edge is updated every Θ(n2) interactions in expectation which coincides with Θ(n) parallel time. This phenomenon provides a natural lower bound on the time complexity for any non-trivial network construction designed for this variant. This is in contrast with the standard population protocol model in which efficient protocols operate in O(poly log n) parallel time. The main focus in this paper is on efficient manipulation of linear structures including formation, self-replication and distribution (including pipelining) of complex information in the adopted model. We propose and analyse a novel edge based phase clock counting parallel time Θ(n log n) in the network constructor model, showing also that its leader based counterpart provides the same time guaranties in the standard population protocol model. Note that all currently known phase clocks can count parallel time not exceeding O(poly log n). The new clock enables a nearly optimal O(n log n) parallel time spanning line construction (a key component of universal network construction), which improves dramatically on the best currently known O(n2) parallel time protocol, solving the main open problem in the considered model [9]. We propose a new probabilistic bubble-sort algorithm in which random comparisons and transfers are allowed only between the adjacent positions in the sequence. Utilising a novel potential function reasoning we show that rather surprisingly this probabilistic sorting (via conditional pipelining) procedure requires O(n2) comparisons in expectation and whp, and is on par with its deterministic counterpart. We propose the first population protocol allowing self-replication of a strand of an arbitrary length k (carrying a k-bit message of size independent of the state space) in parallel time O(n(k + log n)). The pipelining mechanism and the time complexity analysis of the strand self-replication protocol mimic those used in the probabilistic bubble-sort. The new protocol permits also simultaneous self-replication, where l copies of the strand can be created in time O(n(k + log n) log l). Finally, we discuss application of the strand self-replication protocol to pattern matching. Our protocols are always correct and provide time guaranties with high probability defined as 1 - n-η, for a constant η > 0
Fast Space Optimal Leader Election in Population Protocols
The model of population protocols refers to the growing in popularity
theoretical framework suitable for studying pairwise interactions within a
large collection of simple indistinguishable entities, frequently called
agents. In this paper the emphasis is on the space complexity in fast leader
election via population protocols governed by the random scheduler, which
uniformly at random selects pairwise interactions within the population of n
agents.
The main result of this paper is a new fast and space optimal leader election
protocol. The new protocol utilises O(log^2 n) parallel time (which is
equivalent to O(n log^2 n) sequential pairwise interactions), and each agent
operates on O(log log n) states. This double logarithmic space usage matches
asymptotically the lower bound 1/2 log log n on the minimal number of states
required by agents in any leader election algorithm with the running time
o(n/polylog n).
Our solution takes an advantage of the concept of phase clocks, a fundamental
synchronisation and coordination tool in distributed computing. We propose a
new fast and robust population protocol for initialisation of phase clocks to
be run simultaneously in multiple modes and intertwined with the leader
election process. We also provide the reader with the relevant formal
argumentation indicating that our solution is always correct, and fast with
high probability.Comment: 21 pages, 2 figures, published in SODA 2018 proceeding
Increasing brightness in multiphoton microscopy with low-repetition-rate, wavelength-tunable femtosecond fiber laser
Many experiments in biological and medical sciences currently use multiphoton
microscopy as a core imaging technique. To date, solid-state lasers are most
commonly used as excitation beam sources. However, the most demanding
applications require precisely adjusted excitation laser parameters to enhance
image quality. Still, the lag in developing easy-to-use laser sources with
tunable output parameters makes it challenging. Here, we show that manipulating
the temporal and spectral properties of the excitation beam can significantly
improve the quality of images. We have developed a wavelength-tunable
femtosecond fiber laser that operates within the 760 - 800 nm spectral range
and produces ultrashort pulses (below 70 fs) with a clean temporal profile and
high pulse energy (1 nJ). The repetition rate could be easily adjusted using an
integrated pulse picker unit within the 1 - 25 MHz range and without strongly
influencing other parameters of the generated pulses. We integrated the laser
with a two-photon excited fluorescence (TPEF) scanning laser microscope and
investigated the effect of tunable wavelength and reducing the pulse repetition
rate on the quality of obtained images. Using our laser, we substantially
improved the images brightness and penetration depth of native fluorescence and
stained samples compared with a standard fiber laser. Our results will
contribute to developing imaging techniques using lower average laser power and
broader use of tailored fiber-based sources
Menopauza - fakty i kontrowersje
Menopausal period induces some hormonal changes in female
organism. These alterations are responsible for many
illnesses and ailments - e.g. disorders of menstrual cycle,
climacteric symptoms, urine incontinence, connective tissue
lesions, postmenopausal osteopenia and osteoporosis. All
the changes negatively influence quality of life of women.
The principal element of menopausal care in this period is
hormone therapy (HT), alleviating many of the climacteric
aliments. This article is focused also on some controversial
topics concerning the HT action on breast, coronary system
and brain.Okres menopauzy wiąże się ze zmianami hormonalnymi
w organizmie kobiety. Zmiany te są odpowiedzialne za
wystąpienie wielu schorzeń i dolegliwości. Należą do nich
między innymi: zaburzenia cyklu miesiączkowego, objawy
zespołu klimakterycznego, nietrzymanie moczu, zmiany
w tkance łącznej oraz osteopenia i osteoporoza pomenopauzalna.
Wszystkie te zmiany niekorzystnie odbijają się
na jakości życia kobiet. Podstawowym elementem opieki
menopauzalnej w tym okresie jest stosowanie terapii hormonalnej,
znoszącej wiele z wyżej wymienionych dolegliwości.
W niniejszym artykule zawarto również sporne kwestie
dotyczące wpływu tej terapii na sutek, układ krążenia
oraz ośrodkowy układ nerwowy
- …