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  1. Mirrokni, V (Ed.)
    Signal estimation problems with smoothness and sparsity priors can be naturally modeled as quadratic optimization with L0-“norm” constraints. Since such problems are non-convex and hard-to-solve, the standard approach is, instead, to tackle their convex surrogates based on L1-norm relaxations. In this paper, we propose new iterative (convex) conic quadratic relaxations that exploit not only the L0-“norm” terms, but also the fitness and smoothness functions. The iterative convexification approach substantially closes the gap between the L0-“norm” and its L1 surrogate. These stronger relaxations lead to significantly better estimators than L1-norm approaches and also allow one to utilize affine sparsity priors. Inmore »addition, the parameters of the model and the resulting estimators are easily interpretable. Experiments with a tailored Lagrangian decomposition method indicate that the proposed iterative convex relaxations yield solutions within 1% of the exact L0-approach, and can tackle instances with up to 100,000 variables under one minute.« less
    Free, publicly-accessible full text available February 1, 2022
  2. Free, publicly-accessible full text available February 1, 2022
  3. We combine equation of state of dense matter up to twice nuclear saturation density (nsat = 0.16 fm−3 ) obtained using chiral effective field theory (χEFT), and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-byorder calculations up to next-to-next-to-next-to-leading order in the χEFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities, and provide stringent limits on the maximum and minimum radii of ∼ 1.4 M andmore »∼ 2.0 M stars. Including χEFT predictions from nsat to 2 nsat reduces the permitted ranges of the radius of a 1.4 M star, R1.4, by ∼ 3.5 km. If observations indicate R1.4 < 11.2 km, our study implies that either the squared speed of sound c 2 s > 1/2 for densities above 2 nsat, or that χEFT breaks down below 2 nsat. We also comment on the nature of the secondary compact object in GW190814 with mass ' 2.6 M , and discuss the implications of massive neutron stars > 2.1 M (2.6 M ) in future radio and gravitational-wave searches. Some form of strongly interacting matter with c 2 s > 0.35 (0.55) must be realized in the cores of such massive neutron stars. In the absence of phase transitions below 2 nsat, the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by χEFT for the baryon density nB in the range 1−2 nsat. Together they imply that the rapid stiffening required to support a high maximum mass should occur only when nB & 1.5 − 1.8 nsat.« less
  4. Power amplification allows animals to produce movements that exceed the physiological limits of muscle power and speed, such as the mantis shrimp’s ultrafast predatory strike and the flea’s jump. However, all known examples of nonhuman, muscle-driven power amplification involve anatomical structures that store energy from a single cycle of muscular contraction. Here, we describe a nonhuman example of external power amplification using a constructed device: the web of the triangle-weaver spider, Hyptiotes cavatus , which uses energy stored in the silk threads to actively tangle prey from afar. Hyptiotes stretches its web by tightening a separate anchor line over multiplemore »cycles of limb motion, and then releases its hold on the anchor line when insects strike the web. Both spider and web spring forward 2 to 3 cm with a peak acceleration of up to 772.85 m/s 2 so that up to four additional adhesive capture threads contact the prey while jerking caused by the spider’s sudden stop subsequently wraps silk around the prey from all directions. Using webs as external “tools” to store energy offers substantial mechanical advantages over internal tissue-based power amplification due to the ability of Hyptiotes to load the web over multiple cycles of muscular contraction and thus release more stored energy during prey capture than would be possible with muscle-driven anatomical elastic-energy systems. Elastic power amplification is an underappreciated component of silk’s function in webs and shows remarkable convergence to the fundamental mechanical advantages that led humans to engineer power-amplifying devices such as catapults and ballistae.« less
  5. Abstract. This study streamlines modeling of the gas–wall process (GWP) of semivolatile organic compounds (SVOC) by predicting gas–wall equilibrium partitioning constant (𝐾𝑤,𝑖 ) and accommodation coefficient (α𝑤,𝑖) of SVOC(i) using a quantitative structure–activity relationship. PaDEL-Descriptor, software that calculates molecular descriptors, is employed to obtain physicochemical parameters (i.e., hydrogen bond acidity (𝐻𝑑,𝑖), hydrogen bond basicity (𝐻𝑎,𝑖), dipolarity/polarizability (𝑆𝑖), and polarizability (α𝑖)) of SVOC(i). For the prediction of 𝐾𝑤,𝑖, activity coefficients (γw,i) of SVOC(i) to the chamber wall are semiempirically predicted using chamber data in the form of a polynomial equation coupled with the physicochemical parameters. 𝛾𝑤,𝑖 of various SVOCs differ inmore »functionalities and molecular sizes ranging from 100 to 104. We conclude that the estimation of 𝛾𝑤,𝑖 is essential to improve the prediction of 𝐾𝑤,𝑖. To predict the impact of relative humidity (RH) on GWP, each coefficient in the polynomial equation for ln(𝐾𝑤,𝑖) was correlated to RH. Increasing RH enhanced GWP significantly for all polar SVOCs. For example, the predicted 𝐾𝑤,𝑖 of 1-heptanoic acid increased more than three times (from 0.58 to 1.96) by increasing RH from 0.4 to 0.75 due to the reduction in 𝛾𝑤,𝑖. The characteristic time for GWP are estimated using 𝐾𝑤,𝑖 and α𝑤,𝑖 to evaluate the effect of GWP on secondary organic aerosol (SOA) mass. It might be significant in the absence of inorganic aerosol, but insignificant in the presence of electrolytic salts, where aqueous reactions dominate SOA growth.« less