Showing papers by "Polytechnic University of Milan published in 1975"
01 Jan 1975
TL;DR: In this paper, a modified gradient step direction is proposed to improve the performance of relaxation methods for large scale linear problems, which can be shown to be effective for large-scale linear problems.
Abstract: Relaxation methods have been recently shown to be very effective, for some large scale linear problems. The aim of this paper is to show that these procedures can be considerably improved by following a modified gradient step direction.
281 citations
TL;DR: The proposed mathematical algorithms have been designed to obtain the time functions of the variables cited above in an analytical form, and a measurement of the relative indefiniteness induced by experimental errors allows a critical evaluation of the experimental procedure.
219 citations
52 citations
TL;DR: Norbornene reacts at room temperature with π-methallyl nickel chloride giving a red-brown complex in which norbornene is presumably πbonded to nickel as mentioned in this paper.
37 citations
TL;DR: In this paper, the depth of causative bodies may be derived from the power spectrum of their magnetic anomalies, and the spectral method is then applied to real cases; the tapering of the anomalies, the application of nonlinear filters and the effects of anomaly superpositions are investigated.
Abstract: The depth of causative bodies may be derived from the power spectrum of their magnetic anomalies. After a short review of the theoretical basis of the method, several examples of its application to synthetic cases are shown. Disturbing effects due to improper choice of the sampling interval and to anomalies only partially contained in the examined segment of the profile are studied.
The spectral method is then applied to real cases; the tapering of the anomalies, the application of non-linear filters and the effects of anomaly superpositions are investigated.
In conclusion, an appraisal of the method and of its possible practical impact has been given.
21 citations
TL;DR: In this paper, a simple nitrogen pumped dye laser is described which, sequentially in time, generates two pulses, one due to superradiance and the other to laser action.
Abstract: A simple nitrogen pumped dye laser is described which, sequentially in time, generates two pulses, one due to superradiance and the other to laser action The duration of both the pulses can be changed continuously on a nanosecond time scale, down to a few tenths of a nanosecond for the superradiant pulse Pulses as short as 04 nsec have been obtained for the superradiant beam
18 citations
TL;DR: Virus particles, viroplasms and tubular inclusions were never observed in mesophyll parenchyma cells, however, the latter contained altered chloroplasts with taillike protrusions and an accumulation of heavy amounts of starch.
17 citations
TL;DR: In this paper, the convergence of the Deser's method was shown to be sound in the field-theoretic approach than in the usual Riemannian one where there is some arbitrariness (cosmological term, etc.).
Abstract: Gravitational field equations and equations of motion for charged particles are obtained by an iterative method, starting from a Lagrangian density in the ''unrenormalized'' flat space-time. Assuming (a) that the proper masses and charges of the particles are constant and (b) that the gravitational potential is a tensor (no scalar component) in the linearized version (first order in the coupling constant f) we obtain, by imposing consistency to all orders in f, Einstein's theory which is therefore sounder in the field-theoretic approach than in the usual Riemannian one where there is some arbitrariness (cosmological term, etc.). The convergence of our procedure is based on Deser's method but also includes the presence of electromagnetic fields and considers in addition to the energy-momentum tensor T/sub alpha//sub beta/, the most general divergenceless tensor t/sub alpha//sub beta/ depending on four arbitrary parameters which prove to be no longer observable in the ''renormalized'' space. (AIP)
15 citations
TL;DR: In this article, the authors focus on the advances in the chemistry of chrom-3-enes and their derivatives, focusing on the behavior of 2,2-dialkyl chromenes.
Abstract: Publisher Summary This chapter focuses on the advances in the chemistry of chrom-3-enes. No quantum mechanical calculation, or similar theoretical treatment seems to have been dedicated to chrom-3-ene or to its simple derivatives. Interatomic distances and angles of two bromo derivatives of natural chromenes have been obtained by X-ray analysis. The UV absorption, emission, fluorescence, and fluorescence excitation spectra of some 2,2-dialkylchromenes have been studied in connection with their photochromic behavior. Almost every class of natural phenolic compounds contains examples of substances with a 2,2-dialkylchromene ring, and the number of those that are discovered increases every year. It would be difficult to give an exhaustive list of these compounds. Their chemistry is discussed here only when it is related to some particular behavior of the benzopyran ring. Much progress has been made in the last, ten years in the synthesis of chrom-3-enes. Most of the activity has been stimulated by the discovery of new natural products and by the suggestions of possible biosynthetic pathways. In general, the reactions of chromenes are those expected from styrenes. Perhaps for this reason little attention has been paid to their chemical behaviour in recent years. Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances. Chromenes with structure and activity similar to those of hashish constituents have been prepared. A number of chromenes, mostly with aryl substituents in positions 2,3 and 4 have been patented as biologically active substances.
14 citations
TL;DR: In this paper, a lattice dynamics approach to the surface energy γ for brittle fracture of several ionic crystals is presented, based on recent work on surface dynamics and a reformulation of a model previously worked out for metals.
11 citations
TL;DR: In this paper, the hydrogenolysis and skeletal isomerization reactions of n-hexane on Pt-black have been studied as a function of thermal pretreatments of catalysts in different atmospheres.
TL;DR: In this paper, the influence of the size of side groups and the chain extension on the inherent elastic modulus of macromolecular chains in the crystalline state is discussed.
Abstract: Some general considerations are given concerning the conformational factors affecting the inherent elastic modulus of the macromolecular chains in the crystalline state. In particular, the influence of the size of side groups and that of the chain extension are discussed. Some conformational properties concerning two polymers with different stiffness, both containing amide groups, are clarified.
01 Jan 1975
TL;DR: In this paper, perfectly plastic beams and frames in which the bending moment is the only generalized stress (that is the stress-type variable that enters the yield condition) were considered.
Abstract: This paper concerns perfectly plastic beams and frames in which the bending moment is the only generalized stress (that is the stress-type variable that enters the yield condition).
TL;DR: On the basis of physiological knowledge, a mathematical model reproducing the pattern of activity of the acoustic nerve, in response to the most typical acoustic stimuli, is proposed and has been simulated on a digital computer.
Abstract: The peripheral acoustic system can be easily subdivided into three distinct subsystems: a mechanical one (middle ear and basilar membrane), the set of mechanical-to-neural transducers (the inner and outer hair cells) and the neural part. The functional behaviour of the external and middle ear is well known, but the processes concerned with the transduction of mechanical motion into neural activity are still under investigation. Actually the mechanism of transmission of information concerning acoustic stimuli to the central nervous pathways is not clearly known. On the basis of physiological knowledge, a mathematical model reproducing the pattern of activity of the acoustic nerve, in response to the most typical acoustic stimuli, is proposed. This has been simulated on a digital computer.
TL;DR: In this article, the energy-momentum tensor tensor was derived from a variational principle starting from unrenormalized flat space-time, and a field-theoretic approach was applied to coupled gravitational and electromagnetic fields.
Abstract: The deduction of the energy-momentum tensor from a variational principle is clarified in the field-theoretic approach starting from the unrenormalized flat space-time. The procedure is applied to coupled gravitational and electromagnetic fields obtaining the relavant, total energy-momentum tensor to second order in the gravitational coupling constant $f$.
TL;DR: In this article, necessary and sufficient conditions for shakedown in a structure subjected to given histories of external loads and imposed strains, in the presence of significant inertia and damping effects, are established.
Abstract: Two necessary and sufficient conditions for shakedown in a structure subjected to given histories of external loads and imposed strains, in the presence of significant inertia and damping effects, are established. Structural discretization and piecewise linearization of constitutive laws are adopted. This permits to consider several hardening materials and geometric effects, in the spirit of second order theory. The statements can be regarded as a generalization to a broader context of the classical Bleich-Melan and Koiter shakedown theorems. With respect to previous work, the main novelty is the simultaneous coverage of dynamic and second order effects.
TL;DR: In this article, the authors evaluate the conventional losses in rectifier transformers, as performed according to various national standards, and particularly as performed under International Electrotechnical Commission (IEC) recommendations.
Abstract: Evaluation of conventional losses in rectifier transformers, as performed according to the various national standards, and particularly as performed according to International Electrotechnical Commission (IEC) recommendations, is scarcely satisfactory because it does not take into proper consideration the pulsating nature of the current and neglects some elements that give a fair contribution to total losses. Some test elaboration methods are proposed, as well as a test circuit to evaluate in a more accurate way the losses in transformers, interphase transformers, and saturable reactors. Some examples are given, and the available experimental result is in a good agreement with the calculations.
TL;DR: In this paper, the authors presented spectral measurements of the light emitted by a pulsed high-pressure mercury capillary lamp, which was used in a previous work to pump a Rhodamine 6G laser.
Abstract: Calibrated spectral measurements of the light emitted by a pulsed high-pressure mercury capillary lamp are presented. These pulsed lamps have been used in a previous work to pump a Rhodamine 6G laser. The measurements presented here show a very high efficiency in the blue and near UV part of the spectrum, suggesting that these lamps represent a very attractive pumping source for dye lasers emitting in the blue. In particular, as an example, a computation has been made for a basic solution of 4-MU yielding a spectral efficiency of 24%.
TL;DR: In this article, three types of isomerization mechanism and three classes of catalysts have been revealed, and they show an activity in double bond shifting and cistrans isomerisation at low temperatures.
Abstract: Many oxidation catalysts show an activity in double bond shifting and cistrans isomerization at low temperatures. Three types of isomerization mechanism and three classes of catalysts have been revealed.
TL;DR: In this paper, the ESR technique was used to investigate the sorption of Mn2+ on γ-alumina from aqueous solutions, and it was shown that a limited amount (∼10−6 mol/g of γ alumina) is irreversibly adsorbed on lattice defects.
Abstract: The ESR technique was used to investigate the sorption of Mn2+ on γ-alumina from aqueous solutions. The results suggest that Mn2+ is adsorbed in two main forms: 1) a limited amount (∼10−6 mol/g of γ-alumina) is irreversibly adsorbed on lattice defects, and 2) beyond this amount, Mn2+ is reversibly adsorbed as a hydrated complex dispersed on the surface. Upon thermal treatment in air the adsorbed surface species are oxidized while the lattice sites remain unchanged.
TL;DR: In this paper, an extension of the Π-criterion to the case of boundary optimal constant control is presented and a particular case discussed in which the condition above can be given the useful form of a circle criterion.
TL;DR: In this paper, the optimum coupling for dye lasers has been theoretically studied, taking into account the correct rate equations, and the results are applicable also to flashlamppumped dye lasers, provided that the duration of the pumping pulse is long compared to the decay times of the upper singlet band and of the triplet band.
Abstract: The optimum coupling for dye lasers has been theoretically studied, taking into account the correct rate equations The results obtained are applicable also to flashlamppumped dye lasers, provided that the duration of the pumping pulse is long compared to the decay times of the upper singlet band and of the triplet band The behaviour of the output power is also discussed and an expression for the efficiency of the laser is obtained
TL;DR: In this paper, a high pressure Hg capillary lamp (PEK AH 6-2-B) was successfully pulsed to produce laser action in a methanol solution of Rhodamine 6 G.
Abstract: A high pressure Hg capillary lamp (PEK AH 6-2-B) has been successfully pulsed to produce laser action in a methanol solution of Rhodamine 6 G. This lamp may provide a better laser efficiency than that available with a xenon flashtube.
TL;DR: Two configurations of fast operational amplifiers are considered for the implementation of analog circuits in nuclear electronics, for pulse widths shorter than 100 ns, and it is shown how they can be compensated for best operation in ifferent conditions for feedback network and external load.
08 Sep 1975
TL;DR: Subgradient methods are shown to yield very good algorithms for computing tight lower bounds to the solution of many polynomial complete problems.
Abstract: Many polynomial complete problems can be reduced efficiently to three matroids intersection problems. Subgradient methods are shown to yield very good algorithms for computing tight lower bounds to the solution of these problems. The bounds may be used either to construct heuristically guided (branch-and-bound) methods for solving the problems, or to obtain an upper bound to the difference between exact and approx imate solutions by heuristic methods. The existing experience tend to indicate that such bounds would be quite precise.
01 Jan 1975
TL;DR: In this paper, two basic problems in periodic optimization are reviewed: 1) Can the optimal steady-state operation of a system be improved by cycling and 2) What is the best periodic operation when consideration is restrained to high-frequency (relaxed) controls?
Abstract: Two basic problems in Periodic Optimization are shortly reviewed. The first one refers to the question:“Can the optimal steady-state operation of a system be improved by cycling? The second question is: What is the best periodic operation when consideration is restrained to high-frequency (relaxed) controls?” Both these questions are then discussed with reference to a class of singularly perturbed dynamic systems and a somewhat paradoxical result is pointed out.