Lyotropic mesomorphism of alkyl esters of acyl-L-carnitines

The lyotropic polymorphism of a series of alkyl esters of acyl-L-carnitine has been studied by optical polarizing microscopy and X-ray diffraction. The different structures observed as a function of concentration and temperature have been characterized and their topology determined. As a result, two different phase sequence patterns have been detected: esters of normal alcohols bearing an alkyl chain of 6 or more carbon atoms in the acyl substituent display only a lamellar phase, while compounds which bear a relatively short alkyl chain (4 or less carbon atoms) show in addition non-lamellar type I hexagonal and cubic Q²³⁰ phases. From the analysis of the areas-per-molecule at the polar/apolar interface, the ability of the compounds investigated to form not only non-lamellar phases, but also direct micelles in isotropic solution has been related to the structural characteristics of the molecules. Curved, convex interfaces (in micelles and in non-lamellar phases) are possible only for the most polar acylcarnitines which have a relatively short alkyl chain, so that they behave like single chain surfactants; the most paraffinic derivatives, which have a relatively long alkyl chain, are effective double chain surfactants and then generate only quasi-planar interfaces.     

Deviations Induced by Tool Sharpening in the Profile of Three Screw Pump Rotors

Three screw pumps represent an important family of positive displacement rotary pumps. They are quite easy to construct, even if the use of a shaped milling cutter for the rotor machining can cause some problems when the tool is not new. In fact the sharpening of the tool modifies the total geometry of the milling cutter by reducing its diameter. The shape of the single cutter does not change, but the cutter radial position is shifted towards the tool center. This causes deviations in the profile cut and the screws consequently need a long breaking-in. In this paper, the problem is analyzed from a quantitative point of view, using a method that allows us to determine the machining error as a function of the tool-geometry variation. Moreover, some alternative solutions to this problem are suggested.     

Algorithms for railway crew management

Crew management is concerned with building the work schedules of crews needed to cover a planned timetable. This is a well-known problem in Operations Research and has been historically associated with airlines and mass-transit companies. More recently, railway applications have also come on the scene, especially in Europe. In practice, the overall crew management problem is decomposed into two subproblems, called crew scheduling and crew rostering. In this paper, we give an outline of different ways of modeling the two subproblems and possible solution methods. Two main solution approaches are illustrated for real-world applications. In particular we discuss in some detail the solution techniques currently adopted at the Italian railway company, Ferrovie dello Stato SpA, for solving crew scheduling and rostering problems.     

Boundary blow-up problems for Hessian equations

This paper is concerned with large solutions of Hessian equations , in a strictly k-convex domain , i.e. solutions such that as . We prove existence, in the viscosity sense, of this kind of solutions assuming that g satisfies some natural growth conditions. Non-existence and uniqueness results are given too. Received: 18 November 1997     

On Dual Based Lower Bounds for the Sequential Ordering Problem with Precedences and Due Dates

The Sequential Ordering Problem (herewith, SOP) with precedence relationships was introduced in Escudero (1988), and extended to cover release and due dates in Escudero and Sciomachen (1993). It has a broad range of applications, mainly in production planning for manufacturing systems. The problem consists of finding a minimum weight Hamiltonian path on a directed graph with weights on the nodes and the arcs, satisfying precedence relationships among the nodes and given lower and upper bounds on the weights of the Hamiltonian subpaths. In this paper we present a model for the constrained minimum weight Hamiltonian path problem with precedences and due dates forcing constraints, and introduce related valid cuts that can be used in a separation framework for the dual (Lagrangian based) relaxation of the problem. We also provide an heuristic separation procedure to obtain those cuts, so-called the Lagrangian Relax-and-Cut (LRC) scheme. Computational experience is given for variations of some SOP cases already reported in the literature.     

Millimeter-wave investigation, simplified interpretation of the fourfold rotational spectrum, and dynamics of the internal motions of acetaldehyde-argon

The rotational spectra of the acetaldehyde-argon van der Waals complex have been measured by free jet absorption millimeter-wave spectroscopy in the frequency range 60-78 GHz. Each rotational transition is split into four hyperfine component lines, which is evidence of the occurring of two different internal motions. The splittings have been interpreted in terms of a coupled Hamiltonian that precisely determines the separation of energy levels due to the tunneling of the rare gas atom between two equivalent minima, while the information on the barrier to internal rotation of the methyl group is obtained from the pattern of the component A-E lines due to this motion. The interaction of the rare gas atom with the acetaldehyde moiety is reflected in a reduction of the V₃ barrier to internal rotation in going from the molecule to the weakly bound complex of about 20%. The barrier to the Ar tunnelling has been estimated to be 26 cm⁻¹.     

A stochastic control approach to reciprocal diffusion processes

The problem of forcing a nondegenerate diffusion process to a given final configuration is considered. Using the logarithmic transformation approach developed by Fleming, it is shown that the perturbation of the drift suggested by Jamison solves an optimal stochastic control problem. Such perturbation happens to have minimum energy between all controls that bring the diffusion to the desired final distribution. A special property of the change of measure on the path-space that corresponds to the aforesaid perturbation of the drift is also shown.