On Exact and Approximate Solutions for Hard Problems: An Alternative Look

We discuss in an informal, general audience style the da Costa-Doria conjecture about the independence of the P = NP hypothesis and try to briefly assess its impact on practical situations in economics. The paper concludes with a discussion of the Coppe-Cosenza procedure, which is an approximate, partly heuristic algorithm for allocation problems.P vs. NP , allocation problem, assignment problem, traveling salesman, exact solution for NP problems, approximate solutions for NP problems, undecidability, incompleteness

Landau and Ott scaling for the kinetic energy density and the low TcT_c conventional superconductors, Li2Pd3BLi_{2}Pd_{3}B and Nb

The scaling approach recently proposed by Landau and Ott for isothermal magnetization curves is extended to the average kinetic energy density of the condensate. Two low $T_c$ superconductors, Nb and $Li_{2}Pd_{3}B$ are studied and their isothermal reversible magnetization shown to display Landau and Ott scaling. Good agreement is obtained for the upper critical field $H_{c2}(T)$, determined from the Abrikosov approximation for the reversible region (standard linear extrapolation of the magnetization curve), and from the maximum of the kinetic energy curves. For the full range of data, which includes the irreversible region, the isothermal $d.M.B/H^2$ curves for $Li_2Pd_3B$ show an impressive collapse into a single curve over the entire range of field measurements. The Nb isothermal $d.M.B/H^2$ curves exhibit the interesting feature of a constant and temperature independent minimum value

Transition to a Superconductor with Insulating Cavities

An extreme type II superconductor with internal insulating regions, namely cavities, is studied here. We find that the cavity-bearing superconductor has lower energy than the defect-free superconductor above a critical magnetic induction $B^*$ for insulating cavities but not for metallic ones. Using a numerical approach for the Ginzburg-Landau theory we compute and compare free energy densities for several cavity radii and at least for two cavity densities, assuming a cubic lattice of spherical cavities.Comment: 7 pages, 4 figures, to be published in Europhysics Letter

Vanishing of the upper critical field in Bi_2Sr_2CaCu_2O_{8+\delta} from Landau-Ott scaling

We apply Landau-Ott scaling to the reversible magnetization data of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ published by Y. Wang et al. [\emph{Phys. Rev. Lett. \textbf{95} 247002 (2005)}] and find that the extrapolation of the Landau-Ott upper critical field line vanishes at a critical temperature parameter, T^*_c, a few degrees above the zero resistivity critical temperature, T_c. Only isothermal curves below and near to T_c were used to determine this transition temperature. This temperature is associated to the disappearance of the mixed state instead of a complete suppression of superconductivity in the sample.Comment: 3 figure

Stability analysis of bicycles by means of analytical models with increasing complexity

The basic Whipple-Carvallo bicycle model for the study of stability takes into account only geometric and mass properties. Analytical bicycle models of increasing complexity are now available, they consider frame compliance, tire properties, and rider posture. From the point of view of the designer, it is important to know if geometric and mass properties affect the stability of an actual bicycle as they affect the stability of a simple bicycle model. This paper addresses this problem in a numeric way by evaluating stability indices from the real parts of the eigenvalues of the bicycle's modes (i.e., weave, capsize, wobble) in a range of forward speeds typical of city bicycles. The sensitivity indices and correlation coefficients between the main geometric and mass properties of the bicycle and the stability indices are calculated by means of bicycle models of increasing complexity. Results show that the simpler models correctly predict the effect of most of geometric and mass properties on the stability of the single modes of the bicycle. Nevertheless, when the global stability indices of the bicycle are considered, often the simpler models fail their prediction. This phenomenon takes place because with the basic model some design parameters have opposite effects on the stability of weave and capsize, but, when tire sliding is included, the capsize mode is always stable and low speed stability is chiefly determined by weave stability

Analysis of the Compliance Properties of an Industrial Robot with the Mozzi Axis Approach

In robotic processes, the compliance of the robot arm plays a very important role. In some conditions, for example, in robotic assembly, robot arm compliance can compensate for small position and orientation errors of the end-effector. In other processes, like machining, robot compliance may generate chatter vibrations with an impairment in the quality of the machined surface. In industrial robots, the compliance of the end-effector is chiefly due to joint compliances. In this paper, joint compliances of a serial six-joint industrial robot are identified with a novel modal method making use of specific modes of vibration dominated by the compliance of only one joint. Then, in order to represent the effect of the identified compliances on robot performance in an intuitive and geometric way, a novel kinematic method based on the concept of \u201cMozzi axis\u201d of the end-effector is presented and discusse

Research challenges in nanosatellite-DTN networks

Current approaches based on classical satellite communications, aimed at bringing Internet connectivity to remote and underdeveloped areas, are too expensive and impractical. Nanosatellites architectures with DTN protocol have been proposed as a cost-effective solution to extend the network access in rural and remote areas. In order to guarantee a good service and a large coverage in rural areas, it is necessary to deploy a good number of nanosatellites; consequentially, for reliability and load balancing purposes, is also needed a large number of ground stations (or hot spots) connected on the Internet. During a data connection, a server on the Internet that wants to reply to the user on rural area, has many hot spot alternatives to whom it can deliver data. Different hot spots can send data to final destination with different delivery delay depending on the number, position and buffer occupancy of satellites with which it comes into contact. The problem of choosing the optimal hot spot becomes important because a wrong choice could lead a high delivery delay

Experimental observation of high field diamagnetic fluctuations in Niobium

We have performed a magnetic study of a bulk metallic sample of Nb with critical temperature $T_{c}=8.5$ K. Magnetization versus temperature (M {\it vs} T) data obtained for fixed magnetic fields above 1 kOe show a superconducting transition which becomes broader as the field is increased. The data are interpreted in terms of the diamagnetic lowest Landau level (LLL) fluctuation theory. The scaling analysis gives values of the superconducting transition temperature $T_{c}(H)$ consistent with $H_{c2}(T)$% . We search for universal 3D LLL behavior by comparing scaling results for Nb and YBaCuO, but obtain no evidence for universality.Comment: 5 pages, 6 figures, Accepted for publication in Phys.Rev.