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Дмитрий Анатольевич Медведев завел себе видеоблог. Пока там не сделали прием комментариев, их принимает моя коллега Тоня _miscellenia Самсонова

Пятнадцатилетняя девочка в Огайо обвиняется в распространении порнографии, представляющей собой ее же собственные фотки, которые она переслала по мобиле. Теперь ей двадцать лет отмечаться. По утверждению местного прокурора, у него в производстве еще двадцать подобных дел.
Это, впрочем, не рекорд. Есть и шестилетние.

syn_sfreaks

"И поскольку время и пространство взаимосвязаны, вас опять-таки не должно удивлять, что вопрос о путешествиях назад во времени тесно переплетается с проблемой перемещения на сверхсветовых скоростях. Нетрудно показать, что путешествия во времени предполагают сверхсветовые передвижения: сделав последний этап вашего путешествия перемещением назад во времени, вы сможете уложить всю вашу одиссею в сколь угодно короткий срок, а значит, сможете перемещаться с неограниченной скоростью! Но, как мы увидим, верно также и обратное: если вы способны перемещаться с неограниченной скоростью, то сможете и путешествовать назад во времени — одно невозможно без другого." (с) Кратчайшая история времени, Хокинг

Книжка 2005 г. Он серьезно о движении со скоростями выше скоростей света? А что происходит с массой? Про фокусы со временем при рисовании мировой линии знаю, а если более крупные объекты - реально можно сформулировать граничные условия для вселенной, чтобы разрешить сверхсветовые скорости и движение назад во времени?

Сегодня в выпуске: кризис, кризис и снова кризис.

Видеотрансляция с 09.10 до 10.50/

Инджой.

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syn_arxiv_cs

The interaction among users sharing a common transmission channel can be modeled as a non-cooperative game using the game theory framework. When the selfish users choose their transmission probabilities independently without any coordination mechanism, Nash equilibria usually result in a network collapse. We propose a methodology that transforms the non-cooperative game into a Stackelberg game. The Stackelberg equilibrium of the Stackelberg game can overcome the deficiency of the Nash equilibria of the original game. A particular type of Stackelberg intervention is constructed to show that any positive payoff profile feasible with independent transmission probabilities can be achieved as a Stackelberg equilibrium payoff profile. We discuss criteria to select an operating point of the network and informational requirements for the Stackelberg game. We relax the requirements and examine its effects on performance. While the considered transmission scenario is relatively simple, we believe that the proposed analysis can serve as a guideline for designing next generation wireless network protocols.

syn_arxiv_cs

We obtain a new upper bound on the capacity of a class of discrete memoryless relay channels. For this class of relay channels, the relay observes an i.i.d. sequence $T$, which is independent of the channel input $X$. The channel is described by a set of probability transition functions $p(y|x,t)$ for all $(x,t,y)\in \mathcal{X}\times \mathcal{T}\times \mathcal{Y}$. Furthermore, a noiseless link of finite capacity $R_{0}$ exists from the relay to the receiver. Although the capacity for these channels is not known in general, the capacity of a subclass of these channels, namely when $T=g(X,Y)$, for some deterministic function $g$, was obtained in [1] and it was shown to be equal to the cut-set bound. Another instance where the capacity was obtained was in [2], where the channel output $Y$ can be written as $Y=X\oplus Z$, where $\oplus$ denotes modulo-$m$ addition, $Z$ is independent of $X$, $|\mathcal{X}|=|\mathcal{Y}|=m$, and $T$ is some stochastic function of $Z$. The compress-and-forward (CAF) achievability scheme [3] was shown to be capacity achieving in both cases.

Using our upper bound we recover the capacity results of [1] and [2]. We also obtain the capacity of a class of channels which does not fall into either of the classes studied in [1] and [2]. For this class of channels, CAF scheme is shown to be optimal but capacity is strictly less than the cut-set bound for certain values of $R_{0}$. We also evaluate our outer bound for a particular relay channel with binary multiplicative states and binary additive noise for which the channel is given as $Y=TX+N$. We show that our upper bound is strictly better than the cut-set upper bound for certain values of $R_{0}$ but it lies strictly above the rates yielded by the CAF achievability scheme.

syn_arxiv_cs

The points-to problem is the problem of determining the possible run-time targets of pointer variables and is usually considered part of the more general aliasing problem, which consists in establishing whether and when different expressions can refer to the same memory address. Aliasing information is essential to every tool that needs to reason about the semantics of programs. However, due to well-known undecidability results, for all interesting languages that admit aliasing, the exact solution of nontrivial aliasing problems is not generally computable. This work focuses on approximated solutions to this problem by presenting a store-based, flow-sensitive points-to analysis, for applications in the field of automated software verification. In contrast to software testing procedures, which heuristically check the program against a finite set of executions, the methods considered in this work are static analyses, where the computed results are valid for all the possible executions of the analyzed program. We present a simplified programming language and its execution model; then an approximated execution model is developed using the ideas of abstract interpretation theory. Finally, the soundness of the approximation is formally proved. The aim of developing a realistic points-to analysis is pursued by presenting some extensions to the initial simplified model and discussing the correctness of their formulation. This work contains original contributions to the issue of points-to analysis, as it provides a formulation of a filter operation on the points-to abstract domain and a formal proof of the soundness of the defined abstract operations: these, as far as we now, are lacking from the previous literature.

syn_arxiv_cs

In this paper we introduce a new class of codes for over-loaded synchronous wireless CDMA systems which increases the number of users for a fixed number of chips without introducing any errors. In addition these codes support active user detection. We derive an upper bound on the number of users with a fixed spreading factor. Also we propose an ML decoder for a subclass of these codes that is computationally implementable. Although for our simulations we consider a scenario that is worse than what occurs in practice, simulation results indicate that this coding/decoding scheme is robust against additive noise. As an example, for 64 chips and 88 users we propose a coding/decoding scheme that can obtain an arbitrary small probability of error which is computationally feasible and can detect active users. Furthermore, we prove that for this to be possible the number of users cannot be beyond 230.

syn_arxiv_cs

In this paper we wish to introduce a method to reconstruct large size Welch Bound Equality (WBE) codes from small size WBE codes. The advantage of these codes is that the implementation of Maximum Likelihood (ML) decoder for the large size codes is reduced to ML decoder for the core codes. This leads to a drastic reduction of the computational cost of ML decoder. Our method can also be used for constructing large Binary WBE (BWBE) codes from smaller ones. Additionally, we explain that although WBE codes are maximizing the sum channel capacity when the inputs are real valued, they are not necessarily appropriate when the input alphabet is binary. The discussion shows that when the input alphabet is binary, the Total Squared Correlation (TSC) of codes is not a proper figure of merit.

syn_arxiv_cs

We present novel bounds on the capacity of the independent and identically distributed binary deletion channel. Four upper bounds are obtained by providing the transmitter and the receiver with genie-aided information on suitably-defined random processes. Since some of the proposed bounds involve infinite series, we also introduce provable inequalities that lead to more manageable results. For most values of the deletion probability, these bounds improve the existing ones and significantly narrow the gap with the available lower bounds. Exploiting the same auxiliary processes, we also derive, as a by-product, a couple of very simple lower bounds on the channel capacity, which, for low values of the deletion probability, are almost as good as the best existing lower bounds.

syn_arxiv_cs

Let $G_{m \times n}$ be an $m \times n$ real random matrix whose elements are independent and identically distributed standard normal random variables, and let $\kappa_2(G_{m \times n})$ be the 2-norm condition number of $G_{m \times n}$. We prove that, for any $m \geq 2$, $n \geq 2$ and $x \geq |n-m|+1$, $\kappa_2(G_{m \times n})$ satisfies $

\frac{1}{\sqrt{2\pi}} ({c}/{x})^{|n-m|+1} < P(\frac{\kappa_2(G_{m \times n})} {{n}/{(|n-m|+1)}}> x) <

\frac{1}{\sqrt{2\pi}} ({C}/{x})^{|n-m|+1}, $ where $0.245 \leq c \leq 2.000$ and $ 5.013 \leq C \leq 6.414$ are universal positive constants independent of $m$, $n$ and $x$. Moreover, for any $m \geq 2$ and $n \geq 2$, $ E(\log\kappa_2(G_{m \times n})) < \log \frac{n}{|n-m|+1} + 2.258. $ A similar pair of results for complex Gaussian random matrices is also established.

syn_arxiv_cs

The paper presents a new stiffness modelling method for multi-chain parallel robotic manipulators with flexible links and compliant actuating joints. In contrast to other works, the method involves a FEA-based link stiffness evaluation and employs a new solution strategy of the kinetostatic equations, which allows computing the stiffness matrix for singular postures and to take into account influence of the internal forces. The advantages of the developed technique are confirmed by application examples, which deal with stiffness analysis of the Orthoglide manipulator.

syn_arxiv_cs

A method is presented for evaluating authors on the basis of citations. It assigns to each author a citation score which depends upon the number of times he is cited, and upon the scores of the citers. The scores are found to be the components of an eigenvector of a normalized citation matrix. The same method can be applied to citation of journals by other journals, to evaluating teams in a league [1], etc.

syn_arxiv_cs

Cognitive radio (CR) has been proposed as an efficient method to reuse the licensed spectrum. It recognizes the primary (licensed) users' signals and adapts its own to minimize the interference it generates. When perfect channel state information is known at the transmitters, the capacity of CR system can be achieved by utilizing the dirty paper coding (DPC). In this paper, we consider the performance of CR systems under both fast and slow fading channels with only channel statistics known at the transmitters. Due to the limited channel state information, the original DPC fails and is replaced by the so-called linear-assignment Gel'fand-Pinsker coding. By carefully designing the parameters based on this precoding, we show that significant rate gains over naively treating primary users' signals as interference can be obtained for both fast and slow fading scenarios. A nested-lattice based coding/decoding scheme is also proposed to implement this precoding in practice, and it validates our theoretical claims.

syn_arxiv_cs

Main problems of Software Engineering appear as a result of incompatibilities. For example, the quality of organization of production process depends on correspondence with existent resources and on common understanding of project goals by all team members. Software design is another example. Its successfulness rides on architecture's conformity with project's concepts. This is a point of great nicety. All elements should make a single space of interactions. And if the laws of such space are imperfect, missequencing comes and the concept of a software system fails. We must do our best for this not to have happened. To that end having a subtle perception of systems structures is essential. Such knowledge can be based only on a fresh approach to the logical law.

syn_arxiv_cs

In this paper, we examine the CE method in the broad context of Monte Carlo Optimization (MCO) and Parametric Learning (PL), a type of machine learning. A well-known overarching principle used to improve the performance of many PL algorithms is the bias-variance tradeoff. This tradeoff has been used to improve PL algorithms ranging from Monte Carlo estimation of integrals, to linear estimation, to general statistical estimation. Moreover, as described by, MCO is very closely related to PL. Owing to this similarity, the bias-variance tradeoff affects MCO performance, just as it does PL performance.

In this article, we exploit the bias-variance tradeoff to enhance the performance of MCO algorithms. We use the technique of cross-validation, a technique based on the bias-variance tradeoff, to significantly improve the performance of the Cross Entropy (CE) method, which is an MCO algorithm. In previous work we have confirmed that other PL techniques improve the perfomance of other MCO algorithms. We conclude that the many techniques pioneered in PL could be investigated as ways to improve MCO algorithms in general, and the CE method in particular.

syn_arxiv_cs

Consider a random matrix whose variance profile is random. This random matrix is ergodic in one channel realization if, for each column and row, the empirical distribution of the squared magnitudes of elements therein converges to a nonrandom distribution. In this paper, noncrossing partition theory is employed to derive expressions for several asymptotic eigenvalue moments (AEM) related quantities of a large Wishart-type random matrix $\bb H\bb H^\dag$ when $\bb H$ has a random variance profile and is nonergodic in one channel realization. It is known the empirical eigenvalue moments of $\bb H\bb H^\dag$ are dependent (or independent) on realizations of the variance profile of $\bb H$ when $\bb H$ is nonergodic (or ergodic) in one channel realization. For nonergodic $\bb H$, the AEM can be obtained by i) deriving the expression of AEM in terms of the variance profile of $\bb H$, and then ii) averaging the derived quantity over the ensemble of variance profiles. Since the AEM are independent of the variance profile if $\bb H$ is ergodic, the expression obtained in i) can also serve as the AEM formula for ergodic $\bb H$ when any realization of variance profile is available.

syn_arxiv_cs

Linear receivers are considered as an attractive low-complexity alternative to optimal processing for multi-antenna MIMO communications. In this paper we characterize the performance of MIMO linear receivers in two different asymptotic regimes. For fixed number of antennas, we investigate the Diversity-Multiplexing Tradeoff (DMT), which captures the outage probability (decoding block-error probability) in the limit of high SNR. For fixed SNR, we characterize the outage probability for a large (but finite) number of antennas.

As far as the DMT is concerned, we report a negative result: we show that both linear Zero-Forcing (ZF) and linear Minimum Mean-Square Error (MMSE) receivers achieve the same DMT, which is largely suboptimal even though outer coding and decoding is performed across the antennas. We also provide an approximate quantitative analysis of the different behavior of the MMSE and ZF receivers at finite rate and non-asymptotic SNR, and show that while the ZF receiver achieves poor diversity at any finite rate, the MMSE receiver error curve slope flattens out progressively, as the coding rate increases.

When SNR is fixed and the number of antennas grows large, we show that the mutual information at the output of a MMSE or ZF linear receiver has fluctuations that converge in distribution to a Gaussian random variable, whose mean and variance can be characterized in closed form. This analysis extends to the linear receiver case a result that was previously obtained for the optimal receiver. Simulations reveal that the asymptotic analysis captures accurately the outage behavior of systems even with a moderate number of antennas.

syn_arxiv_cs

An L(2,1)-labeling of a graph $G$ is an assignment $f$ from the vertex set $V(G)$ to the set of nonnegative integers such that $|f(x)-f(y)|\ge 2$ if $x$ and $y$ are adjacent and $|f(x)-f(y)|\ge 1$ if $x$ and $y$ are at distance 2, for all $x$ and $y$ in $V(G)$. A $k$-L(2,1)-labeling is an assignment $f:V(G)\to\{0,..., k\}$, and the L(2,1)-labeling problem asks the minimum $k$, which we denote by $\lambda(G)$, among all possible assignments. It is known that this problem is NP-hard even for graphs of treewidth 2, and tree is one of a very few classes for which the problem is polynomially solvable. The running time of the best known algorithm for trees had been $\mO(\Delta^{4.5} n)$ for more than a decade, however, an $\mO(n^{1.75})$-time algorithm has been proposed recently, which substantially improved the previous one, where $\Delta$ is the maximum degree of $T$ and $n=|V(T)|$. In this paper, we finally establish a linear time algorithm for L(2,1)-labeling of trees.

syn_arxiv_cs

The proof of Toda's celebrated theorem that the polynomial hierarchy is contained in $\P^{# P}$ relies on the fact that, under mild technical conditions on the complexity class $C$, we have $\exists C \subset BP \cdot \oplus C$. More concretely, there is a randomized reduction which transforms nonempty sets and the empty set, respectively, into sets of odd or even size. The customary method is to invoke Valiant's and Vazirani's randomized reduction from NP to UP, followed by amplification of the resulting success probability from $1/\poly(n)$ to a constant by combining the parities of $\poly(n)$ trials. Here we give a direct algebraic reduction which achieves constant success probability without the need for amplification. Our reduction is very simple, and its analysis relies on well-known properties of the Legendre symbol in finite fields.

syn_arxiv_cs

We present an algorithm for effectively generating binary sequences which would be rated by people as highly likely to have been generated by a random process, such as flipping a fair coin.

syn_arxiv_cs

In this paper, detection of the primary user (PU) signal in an orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) system is addressed. According to the prior knowledge of the PU signal known to the detector, three detection algorithms based on the Neyman-Pearson philosophy are proposed. In the first case, a Gaussian PU signal with completely known probability density function (PDF) except for its received power is considered. The frequency band that the PU signal resides is also assumed known. Detection is performed individually at each OFDM sub-carrier possibly interfered by the PU signal, and the results are then combined to form a final decision. In the second case, the sub-carriers that the PU signal resides are known. Observations from all possibly interfered sub-carriers are considered jointly to exploit the fact that the presence of a PU signal interferers all of them simultaneously. In the last case, it is assumed no PU signal prior knowledge is available. The detection is involved with a search of the interfered band. The proposed detector is able to detect an abrupt power change when tracing along the frequency axis.

syn_arxiv_cs

The branch group of a strongly controllable group code is a shift group. We show that a shift group can be characterized in a very simple way. In addition it is shown that if a strongly controllable group code is labeled with Latin squares, a strongly controllable Latin group code, then the shift group is solvable. Moreover the mathematical structure of a Latin square (as a translation net) and the shift group of a strongly controllable Latin group code are closely related. Thus a strongly controllable Latin group code can be viewed as a natural extension of a Latin square to a sequence space. Lastly we construct shift groups. We show that it is sufficient to construct a simpler group, the state group of a shift group. We give an algorithm to find the state group, and from this it is easy to construct a stronlgy controllable Latin group code.

Sixty-three years after that first false sunrise in the New Mexico desert, the atomic bomb remains in the news. Just the other day, Gov. Sarah Palin said, "Nuclear weaponry, of course, would be the be-all, end-all of just too many people in too many parts of our planet." Doctor Atomic, John Adams's extraordinary dramatization of the Trinity test of July 1945, opens at the Metropolitan Opera next Monday. Having chronicled the premiere in 2005, I'm most curious to see the new production, and also to hear Adams's score played by the Met orchestra under Alan Gilbert. Thanks to a generous $500,000 donation, the house is offering a slew of prime orchestra seats for $30 as part of its Rush Ticket program; details here.