Initial Blocks Research Articles

Return times, recurrence densities and entropy for actions of some discrete amenable groups

It is a theorem of Wyner and Ziv and Ornstein and Weiss that if one observes the initialk symbolsX 0,…,X k−1 of a typical realization of a finite valued ergodic process with entropyh, the waiting time until this sequence appears again in the same realization grows asymptotically like 2hk [7, 12]. A similar result for random fields was obtained in [8]: in this case, one observes cubes in ℤd instead of initial segments.In the present paper, we describe generalizations of this. We examine what happens when the set of possible return times is restricted. Fix an increasing sequence of sets of possible times {W n } and defineR k to be the firstn such thatX 0,…,X k−1 recurs at some time inW n . It turns out that |W R k | cannot drop below 2hk asymptotically. We obtain conditions on the sequence {W n } which ensure that |W R k | is asymptotically equal to 2hk.We consider also recurrence densities of initial blocks and derive a uniform Shannon-McMillan-Breiman theorem. Informally, ifU k,n is the density of recurrences of the blockX 0,…,X k−1 inX −n ,…,X n , thenU k,n grows at a rate of 2hk, uniformly inn. We examine the conditions under which this is true when the recurrence times are again restricted to some sequence of sets {W n }.The above questions are examined in the general context of finite-valued processes parametrized by discrete amenable groups. We show that many classes of groups have time-sequences {W n } along which return times and recurrence densities behave as expected. An interesting feature here is that this can happen also when the time sequence lies in a small subgroup of the parameter group.

Balanço cátion-aniônico da dieta no metabolismo de cálcio em ovinos

O objetivo deste trabalho foi estudar o efeito do balanço cátion-aniônico da dieta (BCAD) no balanço e concentração sérica de cálcio em ovinos da raça Santa Inês. O delineamento experimental foi em blocos para controlar o efeito do peso inicial dos animais. Foram utilizados cinco tratamentos com níveis crescentes de BCAD (-160, -40, 140, 250 e 500mEq/kg de matéria seca) com cinco repetições em 25 animais. O BCAD influencia o metabolismo de cálcio em ovinos machos da raça Santa Inês. Com a diminuição do BCAD o cálcio sérico aumentou. Dietas aniônicas aumentaram a excreção urinária de cálcio, entretanto a retenção não foi afetada.

Sensitivity to changing stimulus distribution characteristics in auditory categorization

This experiment examined the ability of humans to categorize sounds as a function of changing training distribution characteristics. Participants were presented non-speech sounds randomly sampled from two overlapping distributions. The sounds consisted of 25 narrow-band noise bursts varying in center frequency from 1000 to 1360 Hz. Four different distribution pairs were created by varying the ratio of stimuli in each distribution (i.e., prior probabilities), resulting in different distribution cross-over points or ideal boundaries. Subjects identified the distribution from which each sound was drawn and were provided feedback. The training distributions were switched midsession without alerting the subjects. Performance was tracked by plotting identification functions and noting boundary placement for each individual block. Previously collected data suggested that the ability to learn optimal boundaries was determined in part by the proximity of the distribution boundary to the stimulus midrange point. Results from the current experiment also suggest that distance from the midrange point constrains learning. Subjects were able to learn optimal or near-optimal boundary placement for overlapping distributions only when the ideal boundary was near the midrange point and when those distributions were presented during the initial blocks of training in the experiment. [Work supported by NSF and NIDCD.]

Dynamics of a quantum measurement

We work out an exactly solvable Hamiltonian model which retains all the features of realistic quantum measurements. In order to use an interaction process involving a system and an apparatus as a measurement, it is necessary that the apparatus be macroscopic. This implies treating it with quantum statistical mechanics. The relevant time scales of the process are exhibited. It begins with a very rapid disappearance of the off-diagonal blocks of the overall density matrix of the tested system and the apparatus. Possible recurrences are hindered by the large size of the latter. On a much larger time scale the apparatus registers the outcome: correlations are established between the final values of the pointer and the initial diagonal blocks of the density matrix of the tested system. We thus derive Born's rule and von Neumann's reduction of the state from the dynamical process.

Adaptive initial blocks for improving gradient-match and side-match vector quantization of images

The selection of initial blocks is one of the important issues in finite-state vector quantizers (FSVQs) for images. Conventional FSVQs use blocks located at fixed positions as initial blocks, which then are coded by the codewords in the super codebook. We propose an adaptive-initial-block (AIB) scheme to determine the initial blocks for gradient-match (GM) and side-match (SM) vector quantizers, which are two significant classes of FSVQs. According to the next-state functions used in GMVQs and SMVQs, the blocks with large boundary matching errors in original VQs are selected as the initial blocks. The error propagation effects can be reduced, and the coding performance is significantly improved. The simulation results show that the peak signal-to-noise ratios of coded images can be 2 dB higher than that of conventional GMVQs and SMVQs when the state codebook sizes are much smaller than that of the super codebook.

Enhanced re-habituation of the orienting response of the human event-related potential

Previous studies found the amplitude of the orienting response (OR) of the human event-related potential to decrease with repeated stimulus presentations. This decrease has been suggested to reflect short-term habituation and/or long-term habituation, both of which are learning processes. However, this earlier research failed to provide direct evidence supporting this claim. The present study attempted to show that the OR pattern shares one important feature of habituation: an enhanced response decrement across stimulus-presentation blocks (enhanced re-habituation). Participants received four blocks of 25 auditory stimulus presentations and showed an OR decrement both within (short-term habituation) and across (long-term habituation) blocks. Importantly, the OR decreased more rapidly during later than initial trial blocks, suggesting enhanced re-habituation. The latter result supports the notion that the amplitude decrement reflects an elementary learning process.

Why Firms Adopt Antitakeover Arrangements

Firms going public have increasingly been incorporating antitakeover provisions in their IPO charters, while shareholders of existing companies have increasingly been voting in opposition to such charter provisions. This paper identifies and analyzes possible explanations for this empirical pattern. Specifically, I analyze explanations based on (1) the role of antitakeover arrangements in encouraging founders to break up their initial control blocks, (2) efficient private benefits of control, (3) agency problems among pre-IPO shareholders, (4) agency problems between pre-IPO shareholders and their IPO lawyers, (5) asymmetric information between founders and public investors about the firm's future growth prospects, and (6) bounded attention and imperfect pricing at the IPO stage.I also discuss the policy implications of the possible explanations. Among other things, the analysis implies that researchers should not automatically infer that arrangements adopted in IPO charters are ones that enhance shareholder value. The analysis also indicates that board veto arrangements is unlikely to serve shareholders in companies with dispersed ownership and should not be chosen as a default. The analysis provides some support for limits on contractual freedom at the IPO stage. Finally, the analysis suggests that it might be desirable for corporate law to use sunset strategies, requiring that entrenching arrangements adopted by charter provisions lapse after a certain period unless renewed by a shareholder vote.

On Designs with Pencilwise Orthogonality and Balance

Suppose we have a factorial experiment with m factors where the ith factor has Si levels, i = 1, 2,…, m. The v = mIIi=1 Si level combinations laid out in a v × b block design D with block sizes k1, k2,…, kb and replications ri,r2,…, rv. The C-matrix for the design is given by C = rδ−Nk−δN1 (1) where rδ =diag(r1,r2,…,rv) k−δ=diag(k−11, k−11,….,k−1b) Nv×b = incidence matrix of the design. The reduced normal equations are given by Ct = Q (2) Q is the vector of adjusted treatment total where t1 = {…t(x1,x2,…,xm)…} (3) is the vector of treatment effects arranged lexicographically, t(x1,x2,…., xm) being the treatment effect at the level combination x = (x1, x2,…., xm) x1 = 0, 1, …, Si -1, i = 1(1)m. We know that for δi = 0 or 1 ∀i, such that δ = (δi, δ2,…, δm) ≠ (0 0…0), an interaction Fδ1,1… Fδ2,2 … Fδmm contains (s1 − 1)δ1 (s2 − 1)δ2… (sm − 1)δm = f(δ) orthonormal contrasts among the v treatment effect t(x) which are given by (4)(P δ1 1 ⊗ P δ2 2 ⊗ P δm m )t=F δ Where (5) P δi i = 1 s i (1, 1, …, 1) 1 × s i if δ = 0 = P (s i −1)×s i if δ i = 1 such that 1 s i (1, 1, …, 1) P i is a δi × δi orthogonal matrix and ⊗ denotes Kronecker product of matrices. Fδ = Fδ11 Fδ22 … Fδmm is said to balanced if either Fδ is not estimable or if (6) dispersion (P δ 1 1 ⊗ … ⊗ P δ m m ) t = σ 2 I f(σ) .λ(δ) when Fδ is estimable, where If(δ) is the identity matrix of order f(δ),λ(δ) is a positive constant depending on 6 and t is any solution of (1.2). If from a design all the factorial effects are balanced, then the design is said to be balanced. If the BLUE's of the treatment contrasts belonging to different factorial effects {Fδ}, are uncorrelated then we say that the design has orthogonal factorial structure (OFS). It is known that a connected, proper, equireplicate and binary design is balanced and has OFS if and only if the design is an Extended Group Divisible design. Pioneering works in this direction can be found in Nair and Rao (1948), Shah (1958, 1960), Kurkjian and Zelen (1963), Kshirsagar (1966) and Hinkelmann and Kempthrone (1963)). A comprehensive study in this respect can be found in Gupta and Mukerjee (1989). Now, some efforts have been made to generalise this concept, where the contrasts belonging to a particular effect are classified in some way and designs have been proposed which allow orthogonal estimation with balance, of the effect contrasts belonging to different classes of the effects. Such designs may be called partially balanced factorial designs. One such design was studied in Das and Chatterjee (1999), where the contrasts belonging to factorial effects were classified by introducing groups among the levels of the factors. In the prime-power symmetric case, there is a standard pencilwise division of the factorial effects (Bose and Kishen 1940, Bose 1947). We have considered this division of factorial effects here and characterised designs allowing orthogonal estimation with balance of the treatment contrasts belonging to different pencils. We have called such designs as pencilwise orthogonal and balanced (FOB) designs. We have observed that a design is FOB if and only if its C-matrix is a particular type of linear combination of the Kronecker products of circulant permutation matrices. In particular, a binary, proper and equireplicate design is FOB if and only if it is a PBIB design based on a ( s m − 1) ( s − 1) associate class PBIB association scheme which we have called ‘pencilwise association (PA) scheme’. The association scheme as well as the designs deserve special attention because the designs available so far (Bose and Kishen 1940, Bose 1947) are resolvable designs where the set of blocks forming a replicate forms a disconnected GD design with block size as power of s. Such designs are obtained by cyclical developments of one or more initial blocks. These designs form a sub-class of FOB designs, which need not be resolvable, nor juxtaposition of disconnected GD designs with block sizes as power of s in general.

Two-dimensional algorithm of the density-matrix renormalization group

We propose an approach to implement the density-matrix renormalization group (DMRG) in two dimensions. With this approach the initial blocks of a $L\ifmmode\times\else\texttimes\fi{}L$ lattice are built up directly from the matrix elements of a $(L\ensuremath{-}1)\ifmmode\times\else\texttimes\fi{}(L\ensuremath{-}1)$ lattice and the topological characteristics of two-dimensional lattices are preserved in the iteration of DMRG. By applying it to the spin-1/2 Heisenberg model on both square and triangular lattices, we find that this approach is significantly more efficient and accurate than other two-dimensional DMRG methods currently in use.

Assessing Attention: Comparison of Response-Inhibition and Traditional Continuous Performance Tests

The purpose of this study was to compare a recently developed response-inhibition task (Conners, 1995) to slow- and fast-event-rate versions of the traditional A-X Continuous Performance Test (CPT). Among 146 normal adults, results revealed significant differences between tasks in omission and commission error rates, reaction time, reaction time variability, and responses to critical signals. Effects of environmental noise and participant anxiety also differed for the three tasks. Traditional CPTs produced time-related performance decrements, but the response-inhibition task produced improvement across initial blocks of trials. The response-inhibition task may measure executive control rather than sustained attention, and therefore may represent functions of different brain systems.

Patient satisfaction and effectiveness of lumbar plexus and sciatic nerve block for total knee arthroplasty

The purpose of this study was to evaluate the efficacy of combined lumbar plexus block techniques for total knee arthroplasty. Long-acting local anesthetics were used to ensure adequate intraoperative and postoperative anesthesia and analgesia. All patients undergoing total knee arthroplasty at our institution were offered lumbar plexus block after obtaining informed consent. Patients for study were a continuous group of 87 patients over a 1-year period. A subset of 40 patients was studied for postoperative analgesia effect. All patients were contacted by phone for a satisfaction survey. There were 87 patients who received initial lumbar plexus and sciatic nerve blocks, 78% (68 of 87) of whom had adequate initial blocks. Sixteen patients (22%) required conversion to general anesthesia intraoperatively because of inadequate anesthesia. A subset of patients studied for postoperative analgesia revealed an average time of 13 hours before the first request for supplemental narcotics. There were no complications related to the lumbar plexus block in our study group of patients. There was a 92% overall satisfaction rate with the anesthesia provided by the lumbar plexus block. Lumbar plexus block can be used successfully for total knee arthroplasty. Lumbar plexus block appears to have advantages for early postoperative analgesia, leading to increased patient comfort and satisfaction.

A data warehouse for designed experiments

Methods are described for the efficient representation of designed experiments within information systems. Details to be stored include the physical properties of the units, the underlying structure of the design, the treatments studied in the experiment and their relation to the underlying structure. This relationship can succinctly be described by storing the generator used to construct the design. Possibilities include design keys, alpha arrays and initial blocks of cyclic designs. The same methods that are used to store the generators can be used to form databases containing repertoires of potential designs. Thus they also enable systems to be written to support the experimental process. A particularly convenient framework for such systems is provided by the visual statistical environment generated by the EU STABLE project.

The NGO Paradox: Democratic Goals and Non-democratic Outcomes in Kazakhstan

THE PROMOTION OF LOCAL NON-GOVERNMENTAL ORGANISATIONS (hereafter LNGOs) in the successor states of the Soviet Union and Eastern Europe has increasingly become the focus of international democracy-building efforts, orchestrated through the active involvement of Western non-governmental organisations (hereafter WNGOs). As part of the democratisation process, Western liberal democracies perceive that support for LNGOs serves as the initial building blocks of a civil society. These efforts at democracy building raise two sets of broad questions. First, what is the nature of these efforts? More specifically, what strategies do WNGOs employ to achieve their goals concerning the development of LNGOs in particular and the promotion of democratisation in general? Second, and more importantly, what have been the net results of these efforts thus far? To what extent can we say that, several years into the transition from state-sponsored socialism and communist party rule, LNGOs are evidence of an emerging 'democratic culture'? Are they indeed contributing to the wider process of democratisation in the former Soviet Union?

Pairwise balanced designs from finite fields

Developing previous work by R.C. Bose, R.M. Wilson, M. Greig and the author, we give a general construction for difference families over finite fields. All the DFs constructed before were generated by a single initial base block so that they were uniform. Here we also consider DFs generated by two or more initial base blocks. These DFs give rise to pairwise balanced designs which of course are balanced block designs exactly when the initial base blocks have constant size.

The emotional counting stroop paradigm: a functional magnetic resonance imaging probe of the anterior cingulate affective division

Background: The emotional counting Stroop (ecStroop) functional magnetic resonance imaging (fMRI) activation paradigm was designed to recruit the anterior cingulate affective division (ACad). Methods: Nine normal, healthy male and female subjects (mean age 24.2 years) reported via button press the number of neutral and negative words that appeared on a screen while reaction time and fMRI data were acquired. Results: We observed a) greater ACad activation for negative versus neutral words during initial presentation blocks; b) lower overall ACad signal intensity during task performance (i.e., both negative and neutral words) compared to the baseline fixation condition; and c) no reaction time increase to negative versus neutral words. Conclusions: In a companion study of a cognitive version of the counting Stroop (Bush et al 1998), these same 9 subjects a) activated the more dorsal anterior cingulate cognitive division; b) also showed the overall decrease in ACad signal intensity; and c) demonstrated a reliable reaction time effect. Taken together, these data offer a within-group spatial dissociation of AC function based upon information content (i.e., cognitive vs. emotional) and/or presence of behavioral interference. We propose that the ecStroop will be a useful fMRI probe of ACad function in anxiety disorders.

The description of experimental designs in information systems

Data from agricultural experiments represent a valuable resource that can be used to construct decision support systems or to study such issues as sustainability, climate change or treatment by environment interactions. However, they are usable only if all the relevant details are recorded in a secure and readily accessible way, and this is best achieved using an information system. Details to be stored include the physical properties of the units, the underlying structure of the design, the treatments studied in the experiment and their relation to the underlying structure. This relationship can succinctly be described for many designs by storing the generator used to construct the design. Possibilities include design keys, design splits, alpha arrays and initial blocks of cyclic designs.

A Tool for Partitioning Structured Multiblock Meshes for Parallel Computational Mechanics

This paper discusses the multiblock (MB)-split load bal ancing tool for structured-grid multiblock Navier-Stokes solvers running on parallel computers of MIMD type. Many load-balancing algorithms and tools are available for com putational fluid dynamics solvers using unstructured meshes. Most load-balancing algorithms for block-struc tured meshes only aim at distributing the initial blocks of the mesh over the processors, without splitting any blocks. A more flexible load-balancing technique uses domain decomposition and splits blocks into smaller blocks. One of the problems with this approach is the complexity of updating block connectivity boundary conditions. The three load-balancing algorithms are implemented and some results for realistic test cases are presented.

The influence of the spatial structure on the electronic properties of porous silicon: quantum chemical study

In order to study the electronic properties of porous silicon (por-Si) we have investigated silicon nanoclusters with atomic configurations satisfying the general features of the radial and angular distribution functions of the por-Si structure modelled using the diffusion-limited aggregation model. The electronic structure calculations were performed within the intermediate neglect of differential overlap approximation of the Hartree-Fock-Roothaan scheme, while the restricted configuration interaction was employed in calculating excited states and transition rates. Both the electronic structure and optical transition strength are found to depend strongly on the cluster size and local environment. Importantly, the inclusion of many-electron effects is crucial in determining the transition rates. The low reactivity of some nanoclusters, together with their intermediate size and well-determined electronic properties, make them prospective initial blocks for the manufacturing of optoelectronic materials with desired characteristics.

The Refined Structure of Bacteriophage MS2 at 2·8 Å Resolution

Bacteriophage MS2 is an icosahedral virus with 180 copies of a coat protein forming a shell around a single-stranded RNA molecule. The coat protein subunits form a lattice with the triangulation number T = 3. The coat protein has a fold which is different from the fold of all other viral coat proteins so far known. It consists of a five-stranded β sheet facing the inside of the particle, and a hairpin and two helices on the outside. The crystal structure has been refined at 2·8 Å resolution. The final R -factor was 0·189 for reflections with F>2σ, and the root-mean-square deviation from idealized bond lengths and bond angles was 0·015 Å and 2·9 degrees, respectively. The three chemically identical conformers A, B and C are largely similar. The B conformer has a unique conformation in one loop, which is involved in 5-fold interactions, while the A and C conformers, which are involved in the quasi-6-fold contacts, are similar throughout the structure. One cis-proline has been identified in the B conformer but the corresponding prolines in A and C are of the trans isomer. This residue is conserved within small RNA coliphages and it is proposed that this isomerization enables a less elongated loop (FG) around the 5-fold axis, thus creating a channel. The extensive dimer contact supports the idea of dimers as initial building blocks. An assembly pathway is proposed where five dimers converge into a pentamer and 12 pentamers are linked together with free dimers creating a complete particle.

Block conjugate gradient methods

In this paper a comprehensive theory is attempted of methods of conjugate-gradient type where the matrix of coefficients may be definite, indefinite or nonsymmetric. The theory is based on ‘leveling’ some underlying quadratic function over a linear manifold rather than just a straight line. It is shown that numerical instabilites occur when this quadratic Hessian is indefinite (well-known) or when another matrix employed, often implicitly, in the calculations is indefinite. The use of the various ‘look-ahead’ algorithms in combatting these instabilities is outlined and it is seen that this amounts to a ‘sequential block’ method. It is further shown that many well-known algorithms have ‘simultaneous block’ versions, where the size and composition of the initial blocks are essentially arbitrary. Since the columns of the individual blocks may be generated simultaneously rather than sequentially these methods lend themselves naturally to the techniques of parallel computation.