More Like this

1. A Converse Result on Convergence Time for Opportunistic Wireless Scheduling

   Neely, Michael J. (January 2020, IEEE INFOCOM---International Conference on Computer Communications)

   This paper proves an impossibility result for stochastic network utility maximization for multi-user wireless systems, including multi-access and broadcast systems. Every time slot an access point observes the current channel states and opportunistically selects a vector of transmission rates. Channel state vectors are assumed to be independent and identically distributed with an unknown probability distribution. The goal is to learn to make decisions over time that maximize a concave utility function of the running time average transmission rate of each user. Recently it was shown that a stochastic Frank-Wolfe algorithm converges to utility-optimality with an error of O(log(T)/T ), where T is the time the algorithm has been running. An existing O(1/T ) converse is known. The current paper improves the converse to O(log(T)/T), which matches the known achievability result. The proof uses a reduction from the opportunistic scheduling problem to a Bernoulli estimation problem. Along the way, it refines a result on Bernoulli estimation. 

   more » « less
2. Mechanisms of direct and converse piezoelectricity in ferroelectric polymers

   https://doi.org/10.1016/j.polymer.2025.128290  

   Zhu, Zhiwen; Rui, Guanchun; Allahyarov, Elshad; Zhang, Honghu; Li, Ruipeng; Taylor, Philip L; Zhu, Lei (April 2025, Polymer)

   Within the linear regime of mechanical and electrical responses, it is commonly accepted that direct and converse piezoelectric coefficients should be the same. However, we observed a consistently higher converse d31 (∼54 pm/V) than the direct d31 (∼42 pC/N) for a quenched, stretched, annealed, and electrically poled poly(vinylidene fluoride-co-trifluorethylene) [P(VDF-TrFE)] 52/48 mol.% sample (abbreviated as coP-52/48QSAP). On the contrary, the direct and converse d31 values were the same for coP-65/35QSAP and coP-55/45QSAP. Small-angle X-ray scattering results showed that coP-52/48QSAP had a higher amount of relaxor-like secondary crystals (SCs) in the oriented amorphous fraction (OAF) (SCOAF) than coP-55/45QSAP and coP-65/35QSAP. To explain the experimental observation, we performed molecular dynamics (MD) simulation of the pure PVDF (without TrFE) to estimate direct and converse piezoelectricity for the PVDF OAF. Based on the MD simulation, the direct d31 had a plateau value around 350 pC/N for the transverse (i.e., along the chain direction) strain up to 1 %, whereas the simulated converse d31 could be lower (for electric field E < 0.8 MV/m), equal (for E = 0.8 MV/m), or higher (for E > 0.8 MV/m) than the direct d31, depending on the poling electric field. From the MD simulation, both mechano-electrostriction and electrostatic interaction were identified in the OAF as the driving force for enhanced piezoelectricity in ferroelectric PVDF. When ferroelectric domains were formed in the OAF by electric poling, the simulated converse d31 became higher than the direct d31. Combining both experimental and MD simulation results, the higher converse d31 than direct d31 for coP-52/48QSAP was understood qualitatively. 

   more » « less
3. Disparities in access and association between access to critical facilities during day-to-day and disrupted access as a result of storm extreme weather events

   https://doi.org/10.1016/j.heliyon.2023.e18841  

   Patrascu, Flavia Ioana; Mostafavi, Ali; Vedlitz, Arnold (August 2023, Heliyon)

   This study examines the relationship between households' access to critical facilities day-to-day and during weather-related extreme events. Despite a robust understanding of both day-to-day access and access during disasters, the interplay between the two remains unclear. To bridge this knowledge gap, we propose a novel empirical approach, using a Texas statewide household survey (N = 810). The survey evaluates day-to-day and past events access, exploring the experiences of respondents during multiple recent disasters, rather than focusing on a specific hazard. Using correlation analysis, we examined various access-related factors such as day-to-day trip duration, alternative trip duration, and loss of access during past events. Additionally, we evaluated the association between access-related factors and sociodemographic characteristics such as income, ethnicity, and urban status. The results indicate: (1) daily trip duration to critical facilities is associated with disrupted access during storm events, and (2) disparities persist during both day-to-day times and during extreme events. These results bring new insights to the existing body of knowledge on day-to-day access and access during disasters. The findings provide scientifically grounded evidence to city managers and planners, emphasizing the need for equitable distribution of facilities to enhance access to essential facilities both in daily life and during extreme weather-related events. 

   more » « less
4. A reverse Minkowski theorem

   https://doi.org/10.4007/annals.2024.199.1.1  

   Regev, Oded; Stephens-Davidowitz, Noah (January 2024, Annals of Mathematics)

   We prove a conjecture due to Dadush, showing that if L is an n-dimensional lattice whose sublattices all have determinant at least one, then the sum over all lattice vectors y of e^{-\pi t^2 ||y||^2} is at most 3/2, where t := 10(log n + 2). From this we derive bounds on the number of short lattice vectors, which can be viewed as a partial converse to Minkowski’s celebrated first theorem. We also derive a bound on the covering radius. 

   more » « less
5. Knowledge as a Breaking of Ergodicity

   https://doi.org/10.1162/neco_a_01741  

   He, Yang; Lubchenko, Vassiliy (March 2025, Neural Computation)

   Abstract We construct a thermodynamic potential that can guide training of a generative model defined on a set of binary degrees of freedom. We argue that upon reduction in description, so as to make the generative model computationally manageable, the potential develops multiple minima. This is mirrored by the emergence of multiple minima in the free energy proper of the generative model itself. The variety of training samples that employ N binary degrees of freedom is ordinarily much lower than the size 2N of the full phase space. The nonrepresented configurations, we argue, should be thought of as comprising a high-temperature phase separated by an extensive energy gap from the configurations composing the training set. Thus, training amounts to sampling a free energy surface in the form of a library of distinct bound states, each of which breaks ergodicity. The ergodicity breaking prevents escape into the near continuum of states comprising the high-temperature phase; thus, it is necessary for proper functionality. It may, however, have the side effect of limiting access to patterns that were underrepresented in the training set. At the same time, the ergodicity breaking within the library complicates both learning and retrieval. As a remedy, one may concurrently employ multiple generative models—up to one model per free energy minimum. 

   more » « less