A Hybrid AlGaInAs–Silicon Evanescent Amplifier

We report a hybrid AlGaInAs-silicon evanescent amplifier incorporating a silicon waveguide with a III-V gain medium. The optical mode of the hybrid amplifier is mostly confined to the silicon waveguide and evanescently coupled to the AlGaInAs quantum-well (QW) region where optical gain is provided by electrical current injection. These two different material systems are bonded by low-temperature oxygen plasma assisted wafer bonding at 300 degC. The fabricated device shows 13 dB of maximum chip gain with 11 dBm of output saturation power. Evanescent coupling allows a lower active region confinement factor to provide a higher saturation output power than amplifiers with centered QWs, which is important for applications that require linear amplification     

Temporary Tasks Assignment Resolved

Among all basic on-line load balancing problems, the only unresolved problem was load balancing of temporary tasks on unrelated machines. This open problem existed for almost a decade, see [12]. We resolve this problem by providing an inapproximability result. In addition, a newer open question is to identify the dependency of the competitive ratio on the durations of jobs in the case where durations are known. We resolve this problem by characterizing this dependency. Finally, we provide a PTAS for the off-line problem with a fixed number of machines and show a 2 -inapproximability result for the general case.     

On the complexity of approximating k-set packing

Given a k-uniform hypergraph, the Maximum k -Set Packing problem is to find the maximum disjoint set of edges. We prove that this problem cannot be efficiently approximated to within a factor of unless P = NP. This improves the previous hardness of approximation factor of by Trevisan. This result extends to the problem of k-Dimensional-Matching.     

The Sales Tax Puzzle – Has the law of demand been repealed?

TRA86 removed the deductibility of state and local sales taxes from U.S. personal income taxes. This effectively increased the price to state governments of sales tax revenue relative to income tax revenues. This implies that state reliance on income taxes relative to sales taxes should have increased after TRA86 was implemented. Leading public finance economists investigated this in the period shortly following the reform and invariably found that the predicted substitution of income for sales taxes did not take place. In fact, several studies noted that state dependence on the sales tax increased relative to income taxes – hence the Sales Tax Puzzle. Several experts tried to rationalize their non-findings. These rationalizations are unconvincing. This paper asserts that the puzzle may be resolved in two different ways. The first, seemingly trivial, is that the analysis must incorporate sufficient time so as to allow for relatively complex adjustments. Our contributions here are to anchor this obvious point with a theoretical model related to earlier work of the authors, and an explicit empirical examination of the lag structures of individual states reactions to the comprehensive tax reform. The second demonstrates that the analysis must take into account regional shifts that were taking place in the U.S. during this period. When such compositional shifts involving political and deductibility patterns are explicitly introduced into the model, the paradoxical findings are resolved both in the long and in the short run. Our contribution here is a demonstration that the use of detailed state-level data, unlike any of the earlier work in this area, allows for the inclusion in the analysis of inter-regional shifts in various parameters. An additional contribution is the explicit use and emphasis of the propensity to itemize as an independent variable. The use of this variable ties this area of analysis of post tax reform behavior into a hitherto untapped strand of public-finance literature. The variable is both theoretically interesting, and proves to have experienced compositional shift patterns which help resolve the sales-tax puzzle paradox.     

Adjoint sensitivity analysis and optimization of hysteretic dynamic systems with nonlinear viscous dampers

In this paper we discuss the adjoint sensitivity analysis and optimization of hysteretic systems equipped with nonlinear viscous dampers and subjected to transient excitation. The viscous dampers are modeled via the Maxwell model, considering at the same time the stiffening and the damping contribution of the dampers. The time-history analysis adopted for the evaluation of the response of the systems relies on the Newmark-β time integration scheme. In particular, the dynamic equilibrium in each time-step is achieved by means of the Newton-Raphson and the Runge-Kutta methods. The sensitivity of the system response is calculated with the adjoint variable method. In particular, the discretize-then-differentiate approach is adopted for calculating consistently the sensitivity of the system. The importance and the generality of the sensitivity analysis discussed herein is demonstrated in two numerical applications: the retrofitting of a structure subject to seismic excitation, and the design of a quarter-car suspension system. The MATLAB code for the sensitivity analysis considered in the first application is provided as “Supplementary Material”.     

Simultaneous shape and topology optimization of prestressed concrete beams

This paper presents a new optimization approach for the design of prestressed concrete beams. The prestressing tendon is modeled as a chain of linear segments that transfer point forces to the concrete domain according to the tendon’s angles. The concrete beam is modeled as a discretized continuum following density-based approaches to topology optimization. The shape of the tendon and the topology of the surrounding concrete are optimized simultaneously within a single problem formulation. A special filtering technique is developed in order to ensure that the tendon is covered by concrete, thus shape and topological variables are tightly coupled. Several test cases demonstrate the applicability of the proposed optimization procedure. The deformation of the optimized designs due to external loads is counteracted by the deformation due to prestressing, hence by tuning the force in the tendon the total deformation can approach zero. Consequently, the beams exhibit a compression-only response meaning that the common goal of prestressed concrete design is achieved.     

Optimal search path for service in the presence of disruptions

We consider the following basic search path problem: a customer residing at a node of a network needs to obtain service from one of the facilities; facility locations are known and fixed. Facilities may become inoperational with certain probability; the state of the facility only becomes known when the facility is visited. Customer travel stops when the first operational facility is found. The objective is to minimize the expected total travel distance.