On the Inducibility of Stackelberg Equilibrium for Security Games

Strong Stackelberg equilibrium (SSE) is the standard solution concept of Stackelberg security games. As opposed to the weak Stackelberg equilibrium (WSE), the SSE assumes that the follower breaks ties in favor of the leader and this is widely acknowledged and justified by the assertion that the defender can often induce the attacker to choose a preferred action by making an infinitesimal adjustment to her strategy. Unfortunately, in security games with resource assignment constraints, the assertion might not be valid; it is possible that the defender cannot induce the desired outcome. As a result, many results claimed in the literature may be overly optimistic. To remedy, we first formally define the utility guarantee of a defender strategy and provide examples to show that the utility of SSE can be higher than its utility guarantee. Second, inspired by the analysis of leader's payoff by Von Stengel and Zamir (2004), we provide the solution concept called the inducible Stackelberg equilibrium (ISE), which owns the highest utility guarantee and always exists. Third, we show the conditions when ISE coincides with SSE and the fact that in general case, SSE can be extremely worse with respect to utility guarantee. Moreover, introducing the ISE does not invalidate existing algorithmic results as the problem of computing an ISE polynomially reduces to that of computing an SSE. We also provide an algorithmic implementation for computing ISE, with which our experiments unveil the empirical advantage of the ISE over the SSE.Comment: The Thirty-Third AAAI Conference on Artificial Intelligenc

Planar Metasurfaces Enable High‐Efficiency Colored Perovskite Solar Cells

The achievement of perfect light absorption in ultrathin semiconductor materials is not only a long‐standing goal, but also a critical challenge for solar energy applications, and thus requires a redesigned strategy. Here, a general strategy is demonstrated both theoretically and experimentally to create a planar metasurface absorber comprising a 1D ultrathin planar semiconductor film (replacing the 2D array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector. Guided by derived formulisms, a new type of macroscopic planar metasurface absorber is experimentally demonstrated with light near‐perfectly and exclusively absorbed by the ultrathin semiconductor film. To demonstrate the power and simplicity of this strategy, a prototype of a planar metasurface solar cell is experimentally demonstrated. Furthermore, the device model predicts that a colored planar metasurface perovskite solar cell can maintain 75% of the efficiency of its black counterpart despite the use of a perovskite film that is one order of magnitude thinner. The displayed cell colors have high purities comparable to those of state‐of‐the‐art color filters, and are insensitive to viewing angles up to 60°. The general theoretical framework in conjunction with experimental demonstrations lays the foundation for designing miniaturized, planar, and multifunctional solar cells and optoelectronic devices.A type of macroscopic planar metasurface absorber with light near‐perfectly and exclusively absorbed by the ultrathin semiconductor film is theoretically and experimentally demonstrated via a general strategy. Guided by this strategy, colored perovskite solar cells are further designed to meet all the desired characteristics including high power conversion efficiency, high‐purity, tunability, and angle‐insensitive colors.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146390/1/advs793.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146390/2/advs793-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146390/3/advs793_am.pd

High‐Purity Hybrid Structural Colors by Enhancing Optical Absorption of Organic Dyes in Resonant Cavity

This work presents a novel approach of incorporating an ultrathin dye film into a classic dielectric‐absorber‐dielectric‐metal resonator configuration for generating high‐purity reflective structural colors. Utilizing a thin film of organic dye having the same color as the targeted reflection color as a part of the cavity layer in the structure, its absorption at complementary color wavelengths is significantly enhanced due to the strong cavity resonances, hence reflection at the unwanted wavelengths strongly suppressed, leading to the improved purity of the desired reflective color. This design principle can be applied to create essentially all colors, and is demonstrated by experiment to produce high‐purity blue and red colors. In addition, the fabricated device exhibits outstanding stability under UV exposure without additional protections compared to traditional organic pigments. The proposed method in this work largely simplifies the design process of high‐purity structural colors, which paves the way for more potential applications in various fields.A simple approach that incorporates an ultrathin dye film into a classic dielectric‐absorber‐dielectric‐metal multilayered structure is presented to produce high‐purity reflective colors. The enhanced optical absorptions of the colored dye layer as a result of the strong cavity resonances can effectively suppress the reflection within the unwanted wavelength range, thus significantly improving the purity of the desired reflective colors.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155972/1/adom202000317.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155972/2/adom202000317_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155972/3/adom202000317-sup-0001-SuppMat.pd

Apexification Management of Mandibular Second Premolar with a Blunderbuss Apex and Periapical Lesion of an Adult Patient

Apexification is widely applied in teenager patients but rarely used in the adult population. Instead, artificial apical barrier with mineral trioxide aggregate (MTA) is clinically accepted, and spontaneous apical closure in nonvital immature teeth of adult patients has rarely been encountered while only apexification of the maxillary incisors in adult patients has been reported. The aim of this case report is to share a successful apexification application in the mandibular premolar with a blunderbuss apex and periapical lesion of an adult patient by using calcium hydroxide: radiographically, spontaneous hard tissue barrier has been established, and narrowing canal space and decreasing area of periapical shadow were documented without complications. Taken together, our study indicates that patient\u27s age and tooth position may not be the critical limitation for apexification. © 2019 Chenshuang Li et al