Positron mobility in anthracene

We have measured the positron mobility in a sample of scintillation grade anthracene at two temperatures. We obtain at 300 K: =(26.0±0.9±2.6) cm²V^–1s^–1 and at 77 K: =(33.4±1.1±3.3) cm²V^–1s^–1, where the first error estimate is statistical and the second is systematic. We have also made preliminary measurements on a highly purified sample that yields =(130±3±20) cm² V^–1 s^–1 at 300 K. The data are consistent with the hypothesis that the positron is scattered from both impurities and acoustic phonons in the first sample, and predominantly from photons in the second. It appears that positrons in pure anthracene crystals are delocalized and have a mean free path of about 85 Å at room temperature.     

Designs and Codes: An update

Since the publication in 1992 of ``Designsand their Codes" significant progress has been made in the generalarea of codes coming from designs. This article reviews thisprogress and presents some of the results — including confirmationof certain conjectures made and answers to some of the questionsraised in the book.     

The dynamical properties of Penrose tilings

The set of Penrose tilings, when provided with a natural compact metric topology, becomes a strictly ergodic dynamical system under the action of by translation. We show that this action is an almost 1:1 extension of a minimal action by rotations on , i.e., it is an generalization of a Sturmian dynamical system. We also show that the inflation mapping is an almost 1:1 extension of a hyperbolic automorphism on . The local topological structure of the set of Penrose tilings is described, and some generalizations are discussed.

     

The mathematical structure of Newtonian spacetime: Classical dynamics and gravitation

We give a precise and modern mathematical characterization of the Newtonian spacetime structure (). Our formulation clarifies the concepts of absolute space, Newton's relative spaces, and absolute time. The concept of reference frames (which are timelike vector fields on ) plays a fundamental role in our approach, and the classification of all possible reference frames on is investigated in detail. We succeed in identifying a Lorentzian structure on and we study the classical electrodynamics of Maxwell and Lorentz relative to this structure, obtaining the important result that there exists only one intrinsic generalization of the Lorentz force law which is compatible with Maxwell equations. This is at variance with other proposed intrinsic generalizations of the Lorentz force law appearing in the literature. We present also a formulation of Newtonian gravitational theory as a curve spacetime theory and discuss its meaning.     

On some computational problems in finite abelian groups

We present new algorithms for computing orders of elements, discrete logarithms, and structures of finite abelian groups. We estimate the computational complexity and storage requirements, and we explicitly determine the -constants and -constants. We implemented the algorithms for class groups of imaginary quadratic orders and present a selection of our experimental results. Our algorithms are based on a modification of Shanks' baby-step giant-step strategy, and have the advantage that their computational complexity and storage requirements are relative to the actual order, discrete logarithm, or size of the group, rather than relative to an upper bound on the group order.

     

Korkin-Zolotarev bases and successive minima of a lattice and its reciprocal lattice

Let _i(L), _i(L^*) denote the successive minima of a latticeL and its reciprocal latticeL ^*, and let [b₁,..., b _n ] be a basis ofL that is reduced in the sense of Korkin and Zolotarev. We prove that and, where and _j denotes Hermite's constant. As a consequence the inequalities are obtained forn7. Given a basisB of a latticeL in ^m of rankn andx ^m , we define polynomial time computable quantities(B) and(x,B) that are lower bounds for ₁(L) and(x,L), where(x,L) is the Euclidean distance fromx to the closest vector inL. If in additionB is reciprocal to a Korkin-Zolotarev basis ofL ^*, then ₁(L) _n ^* (B) and.The research of the second author was supported by NSF contract DMS 87-06176. The research of the third author was performed at the University of California, Berkeley, with support from NSF grant 21823, and at AT&T Bell Laboratories.     

Kinetic study of dipivaloylmethane by ozawa method

The lanthanidic complexes of general formula Ln(C₁₁H₁₉O₂)₃ were synthesized and characterized by elementary analysis, infrared absorption espectroscopy, thermogravimetry (TG) and differential scanning calorimetry (DSC). The reaction of thermal decomposition of complexes has been studied by non-isothermal and isothermal TG. The thermal decomposition reaction of complexes began in the solid phase for Tb(thd)₃, Tm(thd)₃ and Yb(thd)₃ and in the liquid phase for Er(thd)₃ and Lu(thd)₃, as it was observed by TG/DTG/DSC superimposed curves. The kinetic model that best adjusted the experimental isothermal thermogravimetric data was the R1 model. Through the Ozawa method it was possible to find coherent results in the kinetic parameters and according to the activation energy the following stability order was obtained: Tb(thd)₃>Lu(thd)₃>Yb(thd)₃>Tm(thd)₃>Er(thd)₃ This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics of asparaginase II fermentation in Saccharomyces cerevisiae ure2dal80 mutant

Although the quality of nitrogen source affects fermentation product formation, it has been managed empirically, to a large extent, in industrial scale. Laboratory-scale experiments successfully use the high-cost proline as a nonrepressive source. We evaluated urea as a substitute for proline in Saccharomyces cerevisiae ure2dal80 fermentations for asparaginase II production as a model system for nitrogen-regulated external enzymes. Maximum asparaginase II levels of 265 IU/L were observed in early stationary-phase cells grown on either proline or urea, whereas in ammonium cells, the maximum enzyme level was 157 IU/L. In all cases, enzyme stability was higher in buffered cultures with an initial pH of 6.5.     

The apparent molal volumes of mixtures of 1:1 electrolytes in aqueous urea solutions: A test of Young's rule

The apparent molal volumes of sodium chloride, sodium bromide, potassium chloride, and potassium bromide were determined for each salt at 2.000m in 8.000m urea-in-water solvent systems at 25°C. In each instance, the volumes were larger than those observed for the same salts in pure water. The volume changes on mixing the six pairs of these salts were measured at a constant ionic strength of 2m in the same urea-water solvent. The results of this test suggest that Young's rule applies equally well in urea-water solvent systems as it does in pure water.     

The use of site symmetry in constructing symmetry adapted functions

It is shown that the application of a projection operator from a given group to a function is equivalent to the successive application of projection operators from factor groups of the starting group to that function. When used with the factor groups representing the site symmetry of a position and the simplest group of interchanges of positions, this concept provides a very simple method for obtaining symmetry adapted linear combinations of basis functions.