Structure Of Variant Research Articles

On integrating direct methods and isomorphous-replacement techniques: triplet estimation and treatment of errors

The method of joint probability distribution functions has been generalized in order to include and treat different sources of error. The probability distributions of the isomorphous pairs (E(p), E(d)) and of the two triples (E(ph), E(pk), E(ph+k), E(dh), E(dk), E(dh+k)) are obtained, on the assumption that the lack of isomorphism and the errors in measurements cumulate on the E(d) variables. The conditional distributions of the two-phase and the three-phase structure invariants are derived, showing how the reliability of the probabilistic estimates depends on the errors.

The joint probability distributions of structure-factor doublets in displacive incommensurately modulated structures and their applicability to direct methods.

In 1993, alternative normalized structure factors for incommensurately modulated structures were defined [Lam, Beurskens & van Smaalen (1993). Acta Cryst. A49, 709-721]. The probability distribution associated with the structure invariants E(-H)E(H')E(H - H') has approximately the same functional form as the Cochran distribution. It was shown, however, that triplet-phase relations are relatively less reliable when satellites are involved [de Gelder, Israël, Lam, Beurskens, van Smaalen, Fu & Fan (1996). Acta Cryst. A52, 947-954]. In the present paper, an alternative approach is presented: instead of studying the distribution of a three-phase invariant, the probability distribution of the phase sum of two first-order satellite reflections (h,k,l,1 and h',k',l',-1) has been derived under the assumption that the phase of the associated main reflection (h + h',k + k',l + l',0) can be calculated from the known main (or averaged) structure. Intensive tests with randomly generated artificial structures and one real structure show a significant improvement of direct-methods phase-sum statistics. Functional similarities with conventional direct methods, employing normalized structure factors and the Cochran distribution, are discussed.

Lithium germanates: Synthesis, crystal-chemical analysis and their structural invariants

The crystal-chemical classification of Li-germanates was performed. Depending on the types of Ge-polyhedra and their interconnections, the Li-gennanates ace divided into 3 groups: the framework MT-structures; the MT condensed structures and T-structures. The structure invariant M2T4 is established in the framework germanates and in Li4Ge3 VI Ge3 VIO12 germanate with M, T condensed structure. The crystals synthesized is in hydrothermal GeO2—LiOH—H2O system are analyzed on the basis of crystal-chemical classification suggested. The representatives of all groups mentioned above are formed in aqua solutions at high temperatures and pressures. The most stable germanate is characterized b the direct condensation of the dimmers M2T4. Germanates with a prevalence of Ge in the structure (M3T2 and MT3) exist in a very small region at low kmperatures and low LiOH concentration, then they are replaced by the framework germanate MT2.

The pointer assertion logic engine

We present a new framework for verifying partial specifications of programs in order to catch type and memory errors and check data structure invariants. Our technique can verify a large class of data structures, namely all those that can be expressed as graph types . Earlier versions were restricted to simple special cases such as lists or trees. Even so, our current implementation is as fast as the previous specialized tools. Programs are annotated with partial specifications expressed in Pointer Assertion Logic, a new notation for expressing properties of the program store. We work in the logical tradition by encoding the programs and partial specifications as formulas in monadic second-order logic. Validity of these formulas is checked by the MONA tool, which also can provide explicit counterexamples to invalid formulas. To make verification decidable, the technique requires explicit loop and function call invariants. In return, the technique is highly modular: every statement of a given program is analyzed only once. The main target applications are safety-critical data-type algorithms, where the cost of annotating a program with invariants is justified by the value of being able to automatically verify complex properties of the program.

Improvement of SAS triple invariant estimates for macromolecular direct-methods phasing

Single-wavelength anomalous dispersion (SAS) data can in principle be phased by direct methods since a priori estimates of the three-phase structure invariants can be computed from these data. The mean phase error of the most reliable triple estimates for a small protein, however, is typically no better than 60 degrees, and does not bode well for applications to larger structures. A procedure is described that can substantially lower the error in these estimates and introduce a larger number of useful triple invariants into the phasing process. The mean phase error of the most reliable triples for a 2.5 A resolution data set from a Pt derivative of a 115-residue protein was reduced from 55 to 25 degrees by this method. It was also possible to identify a significant number of the poorest triple estimates, those with mean phase errors approaching 90 degrees, such that they could be reliably down-weighted or excluded from the phasing process.

Estimation of individual phases from the four-phase structure invariants in the single isomorphous replacement case.

The probability distribution of the four-phase invariants in the case of single isomorphous replacement has been developed to estimate some individual phases. An example of its application to obtain the phases having special values of 0, pi or +/-pi/2 is given for a known protein structure in space group P2(1)2(1)2(1). The phasing procedure includes the determination of starting phases and an iterative calculation. The initial values of starting phases, which are required by the formula, can be obtained from the estimate of one-phase seminvariants and by specifying the origin and enantiomorph. In addition, the calculations lead to two sets of possible phases for each type of reflection by assigning arbitrarily an initial phase value. The present method provides a possibility for the multisolution technique to increase greatly the number of known phases while keeping the number of the trials quite small.

Perovskite-like crystals of the Ruddlesden-Popper series

The layer perovskite-like structures of the so-called Ruddlesden-Popper phases have been analyzed in terms of crystal chemistry. The geometrical boundaries of the existence ranges of these compounds with multilayer stacks are determined and new phases of this type are predicted. The possible variants of related structures are also considered.

The Pointer Assertion Logic Engine

We present a new framework for verifying partial specifications of programs in order to catch type and memory errors and check data structure invariants. Our technique can verify a large class of data structures, namely all those that can be expressed as graph types. Earlier versions were restricted to simple<br />special cases such as lists or trees. Even so, our current implementation is as fast as the previous specialized tools.<br />Programs are annotated with partial specifications expressed in Pointer Assertion Logic, a new notation for expressing properties of the program store. We work in the logical tradition by encoding the programs and partial specifications as formulas in monadic second-order logic. Validity of these formulas<br />is checked by the MONA tool, which also can provide explicit counterexamples to invalid formulas.<br />Other work with similar goals is based on more traditional program analyses, such as shape analysis. That approach requires explicit introduction of an appropriate operational semantics along with a proof of correctness whenever<br />a new data structure is being considered. In comparison, our approach only requires the data structure to be abstractly described in Pointer Assertion Logic.

Performance Benefits of Simultaneous Over Sequential Menus as Task Complexity Increases

To date, experimental comparisons of menu layouts have concentrated on variants of hierarchical structures of sequentially presented menus. Simultaneous menus-layouts that present multiple active menus on a screen at the same time-are an alternative arrangement that may be useful in many Web design situations. This article describes an experiment involving a between-subject comparison of simultaneous menus and their traditional sequential counterparts. A total of 20 experienced Web users used either simultaneous or sequential menus in a standard Web browser to answer questions based on U.S. Census data. Our results suggest that appropriate use of simultaneous menus can lead to improved task performance speeds without harming subjective satisfaction measures. For novice users performing simple tasks, the simplicity of sequential menus appears to be helpful, but experienced users performing complex tasks may benefit from simultaneous menus. Design improvements can amplify the benefits of simultaneous menu layouts.

Anatomic Considerations in the Treatment of Extracranial Cerebrovascular Disease

This article outlines the anatomic relationships and variants of structures encountered during surgical approaches for the treatment of extracranial cerebrovascular disease.

Hydrodynamics and heat transfer in cooling systems with intersecting channels. 3. Effect of the angle of flow and the number of stages

Results of further investigation of some variants of wafer structures are given. The effect of the angle of flow on the hydraulic resistance and heat transfer at Re numbers of 1·102–2·104 is revealed. The temperature field of a two-stage system is analyzed. It is shown that a size reduction of the structure and an increase in the number of stages make it possible to obtain the maximum possible coefficient of reduced heat transfer (2.5–2.8)·105 W/(m2·K).

Comparison of K-Theory Galois Module Structure Invariants

AbstractWe prove that two, apparently different, class-group valued Galoismodule structure invariants associated to the algebraic K-groups of rings of algebraic integers coincide. This comparison result is particularly important in making explicit calculations.

On Technomorphic Modelling and Classification of Biological Joints

Biological motion systems are of particular interest to engineers in robotics, prosthetics and micromechanics. Since biological motion systems show a high degree of mobility, smooth movements and minimal deployment of material, the analysis of such systems might help to invent or optimize technical motion systems. To enable the transfer of explanatory techniques, biomechanics and engineering need a shared terminology. Generally, a reference limb, muscles, tendons, a joint and a driven limb are forming two closed mechanisms with different transfer functions. The direction of the forces applied to links is influenced by guiding structures. Movable connections can be constructed by form closure, force closure, and compliance of an anisotropic segment between two rigid segments. Eleven different basic variants of rigid-body joint structures can be classified by the possible relative translatory and rotatory movements in orthogonal co-ordinates. If classified by the form of their rigid parts, biological rigid-body joints (diarthroses) show some similarities to technical joints, but occur in fewer basic variants. The functioning of exoskeletal joints can involve hydrostatic forces, depending on the structure of the rigid elements and the type of linkage between them.

Optimal synthesis methodology of nonhomogeneous structures under the influence of electromagnetic waves

The problem of the optimal synthesis of inhomogeneous structures of a discrete set of materials under wave influence is considered. It is established that in the interdependence between parameters of structures that realize the limiting possibilities some order or inner symmetry may exist under certain conditions. This fact allows to solve the problem of synthesis of layer structures in a straightforward way. The problem of a more effective control of energy characteristics of electromagnetic waves is con- sidered on the basis of choice of a structure of inhomogeneous medium. The problem of obtaining the structure of inhomogeneous media, that realize the limiting possibilities of control of energy charac- teristics of electromagnetic waves, is of considerable interest. This problem applies to the problem of multivariant choice. It is shown that in the interdependence between parameters of structures, realizing the control of energy characteristics of electromagnetic waves, qualitative regularities exist. This fact provides the effective selection of a complete totality of the variants of structures that realize the limiting possibilities.

Osseous anatomic variants of the wrist: findings on MR imaging.

A natomic variants of the osseous structures of the wrist are common and readily detected on MR imagmg. Noteworthy variations include accessory ossicles, carpal coalitions, variable lunate morphology, articular lipping at the pisotriquetral joint, and pseudosubluxation of the distal radioulnar joint. Although many of these entities can be identified or clarified on radiographs alone, their appearance on radiography may be ambiguous or radiographs may not be available at the time of MR image evaluation. The correct identification ofthese variants on MR studies is important so that these often incidental entities are not mistaken for pathologic findings or otherwise cause confusion. In some cases, however, these anatomic variants are associated with symptomatic conditions, and familiarity with the MR appearance of these osseous vanants may prevent misinterpretation.

Use of quantitative convergent-beam electron diffraction in materials science.

Methods for quantitative convergent-beam electron diffraction are outlined and some results of our applications of convergent-beam electron diffraction are shown, with emphasis on quantitative analysis of crystal structures in materials science. Examples of thickness measurements and determination of lattice parameters are presented. Measurements of low-order structure factors to obtain information on bonding charge-density distributions are reviewed, with examples from TiAl intermetallics. For non-centrosymmetric crystals, a method to determine three-phase structure invariants is given. Determination of polarity is also discussed.

Three-phase structure invariants and structure factors determined with the quantitative convergent-beam electron diffraction method

Quantitative convergent-beam electron diffraction is used to determine structure factors and three-phase structure invariants. The refinements are based on centre-disc intensities only. An algorithm for param-eter-sensitive pixel sampling of experimental intensities is implemented in the refinement procedure to increase sensitivity and computer speed. Typical three-beam effects are illustrated for the centrosymmetric case. The modified refinement method is applied to determine amplitudes and three-phase structure invariants in noncentrosymmetric InP. The accuracy of the results is shown to depend on the choice of the initial parameters in the refinement. Even unrealistic starting assumptions and incorrect temperature factor lead to stable results for the structure invariant. The examples show that the accuracy varies from 1 to 10 degrees in the electron three-phase invariants determined and from 0.5 to 5% for the amplitudes. Individual phases could not be determined in the present case owing to spatial intensity correlations between phase-sensitive pixels. However, for the three-phase structure invariant, stable solutions were found.

Probability distribution of the four-phase structure invariants of isomorphously related structure factors and its applications.

The probability distribution of the four-phase structure invariants (4PSIs) involving four pairs of structure factors is derived by integrating the direct methods with isomorphous replacement (IR). A simple expression of the reliability parameter for 16 types of invariant is given in the case of a native protein and a heavy-atom derivative. Test calculations on a protein and its heavy-atom derivative using experimental diffraction data show that the reliability for 4PSI estimates is comparable with that for the three-phase structure invariants (3PSIs), and that a large-modulus invariants method can be used to improve the accuracy.

Confirmatory factor analysis of the Parental Bonding Instrument in a Japanese population.

There is controversy surrounding the factor structure of the Parental Bonding Instrument (PBI), a widely used instrument for assessing perceived parental rearing behaviours. Recent studies have proposed five different factor structures, including Parker et al.'s original two-factor model. Four hundred and eighteen employed Japanese adults filled out the PBI. Maximum likelihood confirmatory factor analyses were performed to compare the five different factor structures in terms of model-fit. Parker's original two-factor structure fitted the data poorly. In general, three-factor structures showed better fit. Among the three-factor structures, Murphy's model and Kendler's model were superior (the adjusted goodness-of-fit index > 0.8), with the latter providing the best fit to the data (the goodness-of-fit index > 0.9). When considering invariance of factor structure across gender subgroups and across age subgroups, only Kendler's model was acceptable. Parker's two-factor structure of the PBI may not be appropriate for assessing perceived parental rearing behaviours in a Japanese population. Three-factor structures, in particular Murphy's model and Kendler's model, are preferable. Kendler's model provided the best fit to the data and was relatively invariant across the subgroups in this study. Thus, Kendler's model might prove to be very important for obtaining a factor structure invariant across different cultures.

On Galois structure invariants associated to Tate motives

We establish the equivalence of two definitions of invariants measuring the Galois module structure of K-groups of rings of integers in number fields (due to Chinburg et al. on the one hand and the authors on the other). We also make some remarks concerning the possibility of yet another such definition via Lichtenbaum complexes.