Showing papers by "Charles University in Prague published in 1991"

Efficient partition trees

Abstract: We prove a theorem on partitioning point sets inEd (d fixed) and give an efficient construction of partition trees based on it. This yields a simplex range searching structure with linear space,O(n logn) deterministic preprocessing time, andO(n1?1/d(logn)O(1)) query time. WithO(nlogn) preprocessing time, where ? is an arbitrary positive constant, a more complicated data structure yields query timeO(n1?1/d(log logn)O(1)). This attains the lower bounds due to Chazelle [C1] up to polylogarithmic factors, improving and simplifying previous results of Chazelleet al. [CSW]. The partition result implies that, forrd≤n1??, a (1/r)-approximation of sizeO(rd) with respect to simplices for ann-point set inEd can be computed inO(n logr) deterministic time. A (1/r)-cutting of sizeO(rd) for a collection ofn hyperplanes inEd can be computed inO(n logr) deterministic time, provided thatr≤n1/(2d?1).

Approximations and optimal geometric divide-and-conquer

Abstract: We give an efficient deterministic algorithm for computing ?-approximations and ?-nets for range spaces of bounded VC-dimension. We assume that an n-point range space ? = (X, R) of VC-dimension d is given to us by an oracle, which given a subset A ? X, returns a list of all distinct sets of the form A ? R; R ? R (in time O(|A|d+1)). Given a parameter r, the algorithm computes a (1/r)-approximation of size O(r2 log r) for ?, in time O(n(r2 log r)d). A (1/r)-net of size O(r log r) can be computed within the same time bound. We also obtain a new deterministic algorithm which for a given collection H of n hyperplanes in Ed and a parameter r ? n computes a (1/r)-cutting of (asymptotically optimal) size O(rd). For r ? n1??, where ? > 0 is arbitrary but fixed, the running time is O(nrd?1), which is optimal for geometric divide-and-conquer applications.

Preparative, analytical and fluorescence spectroscopic studies of sulphonated aluminium phthalocyanine photosensitizers

Abstract: Fluorescence spectroscopic studies were carried out on aluminium phthalocyanine with defined numbers (mono, di, tri and tetra) of sulphonate groups. Selective sulphonation was achieved using one of two synthetic methods to prepare a mixture of components which were separated using reverse-phase liquid chromatography. Fluorescence lifetimes were measured in methanol and buffer solution using time-correlated single-photon counting with picosecond laser excitation; the lifetime shows little variation with the number of sulphonate groups. Using steady state excitation, fluorescence quantum yields were determined for the tetrasulphonated component (phi F = 0.51) and, for comparison, unsulphonated aluminium phthalocyanine.

Dynamic Half-Space Range Reporting and Its Application

Abstract: \indent We consider the half-space range reporting problem: Given an $n$ point set $S$ in ${\bf R} ^d$, preprocess it into a data structure, so that, given a query half-space $\gamma$, the points of $S \cap \gamma$ can be reported efficiently. We extend previously known static solutions to dynamic ones, supporting insertions and deletions of points of $S$. For given $m, n \leq m \leq n ^ {\lfloor d/2 \rfloor}$ and an arbitrarily small positive constant $\varepsilon$, we achieve $O(m^{1+ \varepsilon})$ space and preprocessing time, $O \frac {n}{m^{1/ \lfloor d/2 \rfloor}} \, \mbox {log} \, n)$ query time and $O(m^{1 + \varepsilon} / n)$ amortized update time $(d \geq 3)$. We present, among others, the following applications: an $O(n ^ {1 + \varepsilon})$ time algorithm for computing convex layers in ${\bf R}^3$, and an output sensitive algorithm for computing a level in an arrangements of planes in ${\bf R} ^3$, whose time complexity is $O((b+n) \, n^ \varepsilon)$, where $b$ is the size of the level.

Finite-size scaling for Potts models

Abstract: Recently, Borgs and Kotecký developed a rigorous theory of finite-size effects near first-order phase transitions. Here we apply this theory to the ferromagneticq-state Potts model, which (forq large andd⩾2) undergoes a first-order phase transition as the inverse temperatureβ is varied. We prove a formula for the internal energy in a periodic cube of side lengthL which describes the rounding of the infinite-volume jumpΔE in terms of a hyperbolic tangent, and show that the position of the maximum of the specific heat is shifted byΔβ m (L)=(Inq/ΔE)L −d +O(L −2d ) with respect to the infinite-volume transition pointβ t . We also propose an alternative definition of the finite-volume transition temperatureβ t (L) which might be useful for numerical calculations because it differs only by exponentially small corrections fromβ t .

Cutting hyperplane arrangements

Abstract: We consider a collectionH ofn hyperplanes in Ed (where the dimensiond is fixed). An ?-cutting forH is a collection of (possibly unbounded)d-dimensional simplices with disjoint interors, which cover all Ed and such that the interior of any simplex is intersected by at most?n hyperplanes ofH. We give a deterministic algorithm for finding a (1/r)-cutting withO(rd) simplices (which is asymptotically optimal). Forr≤n1??, where ?>0 is arbitrary but fixed, the running time of this algorithm isO(n(logn)O(1)rd?1). In the plane we achieve a time boundO(nr) forr≤n1??, which is optimal if we also want to compute the collection of lines intersecting each simplex of the cutting. This improves a result of Agarwal, and gives a conceptually simpler algorithm. For ann point setX⊆Ed and a parameterr, we can deterministically compute a (1/r)-net of sizeO(rlogr) for the range space (X, {X ? R; R is a simplex}), In timeO(n(logn)O(1)rd?1+rO(1)). The size of the (1/r)-net matches the best known existence result. By a simple transformation, this allows us to find ?-nets for other range spaces usually encountered in computational geometry. These results have numerous applications for derandomizing algorithms in computational geometry without affecting their running time significantly.

Stability and sensitivity-analysis for stochastic programming

Abstract: Stability and sensitivity studies for stochastic programs have been motivated by the problem of incomplete information about the true probability measure through which the stochastic program is formulated and in connection with the development and evaluation of algorithms. The first part of this survey paper briefly introduces and compares different approaches and points out the contemporary efforts to remove and weaken assumptions that are not realistic (e.g., strict complementarity conditions). The second part surveys recent results on qualitative and quantitative stability with respect to the underlying probability measure and describes the ways and means of statistical sensitivity analysis based on Gâteaux derivatives. The last section comments on parallel statistical sensitivity results obtained in the parametric case, i.e., for probability measures belonging to a parametric family indexed by a finite dimensional vector parameter.

A series of 14 new monoclonal antibodies to keratins: Characterization and value in diagnostic histopathology

Abstract: A series of 14 new mouse monoclonal antibodies (MAbs) to keratins is described and the data suggesting their potential value in the differential diagnosis of human tumours are reported. The specificities of individual MAbs of the 'C-series' presented here range from monospecificity for keratin No. 7 (MAbs C-18, C-35, C-62, and C-68), keratin No. 8 (MAbs C-15, C-43, and C-15), and keratin No. 18 (MAbs C-04 and C-08) up to the broadly reacting 'pan-keratin' MAb C-11, with the target epitopes of the remaining four MAbs being shared by different pairs of keratin polypeptides. The results of the biochemical characterization of the MAbs, together with their immunohistochemical staining patterns on frozen as well as on paraffin sections of normal human tissues, suggest that they represent a significant contribution to the growing list of anti-keratin MAbs applicable in both research and routine diagnostic pathology. The immunohistochemical examination of a wide range of human neoplasms with the new MAbs not only confirmed their value in making distinctions between carcinomas, on the one hand, and lymphomas, and gliomas, on the other, but also verified the possibility of more subtle subdivisions within the group of adenocarcinomas and their metastases. Furthermore, the identification of small subsets of breast carcinomas with decreased levels or apparent loss of the keratin No. 7 polypeptide and some cases of stomach carcinoma with apparently induced expression of this keratin suggests that such 'exceptions' must be considered when using keratin spectra as one of the criteria in differential diagnosis.

Medial vs lateral frontal lobe lesions and differential impairment of central-gaze fixation maintenance in man.

Abstract: SUMMARY The ability to maintain central-gaze fixation was studied in 35 patients with unilateral frontal lobe lesions. The stability of central-gaze fixation was tested in 2 oculomotor tasks requiring the suppression of reflexive saccades triggered by the sudden appearance of novel, peripheral visual stimuli. In the first task(‘Instructed’), maintenance of gaze fixation was based on verbal instruction, whereas, in the second task (‘Non-instructed’), it was based upon a foveating mechanism maintained by sensory stimulation during the performance of a categorization task. Patients with frontal lobe lesions were classified into 3 groups: a ventrolateral ( n = 18), a dorsolateral ( n = 10) and a medial ( n = 7) group. The control group consisted of 20 healthy subjects. In the Non-instructed task, no group differences were found in the rate of reflexive saccades. Thus, lesions of the frontal lobe did not affect reactivity to extraneous stimuli per se . The only exception to this rule was the group of patients with ventrolateral lesions in the left frontal lobe, in which a higher number of reflexive saccades directed to the contralateral hemifield was observed. In the Instructed task, 2 different patterns of deficit in suppression of reflexive saccades were found. First, a bidirectional deficit was noted in the ventrolateral group, which might be attributed to an impairment in either maintaining a selected program of behaviour ‘on-line’ or suppressing inappropriate alternatives interfering with that program. Second, a unidirectional deficit in voluntary suppression of reflexive saccades triggered from within the contralateral hemifield was observed in the medial group. It is suggested that this type of deficit may reflect malfunctioning of the supplementary motor area-anterior cingulate system involved in the suppression of externally triggered interfering programs. Finally, no significant impairment was found in the dorsolateral group.

Evolution of nappes in the eastern margin of the Bohemian Massif: a kinematic interpretation

Abstract: The entire pile of nappes in the eastern margin of the Bohemian massif is characterized by two stages of Variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution.

Generalized Stein's model for anatomically complex neurons.

Abstract: A neuron with a large dendritic structure is considered. The number of synapses located on the dendrites is substantially higher than on the soma. The synaptic input effect on the neuronal excitability decreases with distance between a synapse ending and the trigger zone. Two areas are distinguished in accordance with the effect of synaptic input — dendritic and somatic. The dendritic area, when compared to the soma, is characterized by much higher intensity of its activation but the amplitudes of synaptically evoked changes of the membrane potential at the trigger zone are in general small. This situation is suitable for a diffusion approximation. However, on the soma, especially in the proximity of the trigger zone, the membrane potential changes are a large fraction of the threshold depolarization. The membrane potential at the trigger zone is modelled by a one-dimensional stochastic process. The diffusion Ornstein-Uhlenbeck process serves as a basis of the model; however, at the moments of somatic synapses activation its voltage changes in jumps. Their sizes represent the amplitudes of the evoked postsynaptic potentials. The unimodal histograms of interspike intervals can be explained by the model. The values of the coefficient of variation greater than one are connected with substantial inhibition.

Spherical harmonic expansion of the Earth's crustal thickness up to degree and order 30

Abstract: The knowledge of the depth of the Mohorovicic' discontinuity (Moho) is important for investigating the 3-D structure of the Earth's interior, since the variations in the crustal thickness can significantly affect geophysical observations made at the Earth's surface. In this paper, the information about the topography of the crust-mantle boundary is compiled from various sources and the crustal thickness is represented in a spherical harmonic expansion up to degree and order 30.

Sexuality of women with anorexia nervosa.

Abstract: The sexual development and life of 30 adult women with anorexia nervosa and of 50 control women was investigated using a structured interview and 3 sexological questionnaires. Heterosexual development was found to be normal in the initial stages, but psychosexual adaptation in adulthood was impaired. Primary or secondary insufficiencies of sexual life were found for 80% of the anorectic patients. Biological and psychosocial factors may participate in this unfavourable state. Discussion of sexual problems should be part of the process of treating women with anorexia nervosa.

Stable isotope dilution analysis of N-acetylaspartic acid in CSF, blood, urine and amniotic fluid: accurate postnatal diagnosis and the potential for prenatal diagnosis of Canavan disease.

Abstract: textA sensitive and selective analytical technique is described for the determination of N-acetylaspartic acid in body fluids using stable isotope dilution in combination with positive chemical ionization mass spectrometry with selected ion monitoring. Control mean and ranges have been established: in urine 19.5 and 6.6-35.4 μmol/mmol creat.; in plasma 0.44 and 0.17-0.81 μmol/L; in cerebrospinal fluid 1.51 and 0.25-2.83 μmol/L; and in amniotic fluid 1.27 and 0.30-2.55 μmol/L. In a patient with Canavan disease, N-acetylaspartic acid concentration was elevated 80-fold in urine and 20-fold in plasma compared to the control means. A subsequent pregnancy of the mother was monitored and the N-acetylaspartic acid concentration in the amniotic fluid was within the control range and a healthy child was born.