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  1. A self-excited system is a nonlinear system with the property that a constant input yields a bounded, nonconvergent response. Nonlinear identification of self-excited systems is considered using a Lur'e model structure, where a linear model is connected in feedback with a nonlinear feedback function. To facilitate identification, the nonlinear feedback function is assumed to be continuous and piecewise affine (CPA). The present paper uses least-squares optimization to estimate the coefficients of the linear dynamics and the slope vector of the CPA nonlinearity, as well as mixed-integer optimization to estimate the order of the linear dynamics and the breakpoints of the CPA function. The proposed identification technique requires only output data, and thus no measurement of the constant input is required. This technique is illustrated on a diverse collection of low-dimensional numerical examples as well as data from a gas-turbine combustor. 
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  2. Otitis media (OM) is the most common disease among young children and one of the most frequent reasons to visit the pediatrician. Development of OM requires nasopharyngeal colonization by a pathogen which must gain access to the tympanic cavity through the eustachian tube (ET) along with being able to overcome the defense mechanisms of the immune system and middle ear mucosa. OM can be caused by viral or bacterial infection. The three main bacterial pathogens are Streptococcus pneumoniae, nontypeable Haemophilus influenzae (NTHi), and Moraxella catarrhalis. Innate immunity is important in OM resolution as the disease occurs in very young children before the development of specific immunity. Elements of innate immunity include natural barriers and pattern recognition receptors such as Toll like receptors (TLRs), and Nod like receptors (NLRs). Surfactant proteins A (SP-A) and D (SP-D) act as pattern recognition receptors and are found in the lung and many other tissues including the ET and the middle ear where they probably function in host defense. Surfactant has a potential for use in the treatment of OM due to surface tension lowering function in the ET, and the possible immune functions of SP-D and SP-A in the middle ear and ET. 
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  3. Abstract

    Elevated deposition of atmospheric nitrogen (N) has shifted nutrient availability in terrestrial and aquatic habitats of ecosystems, but rarely are ecosystem processes in those components examined simultaneously. We used a multi-decadal, whole, paired watershed experiment to determine how chronic N enrichment with (NH4)2SO4alters litter decomposition in terrestrial and stream systems. We also used short-term phosphorus (P) enrichment experiments within both watersheds to determine whether chronic N enrichment enhances P limitation of decomposition. Leaves from N-treated and reference watersheds were used in a reciprocal design to parse effects of altered nutrient availability in leaves and in the environment. We found divergent responses of terrestrial and stream decomposition to altered nutrient regimes. Chronic experimental N enrichment increased N and P concentrations in post-abscission leaves which decayed faster than leaves from the reference watershed in the terrestrial environment. Experimental N enrichment also did not induce P limitation of terrestrial decomposition. In contrast, litter decomposition rate in the two streams was not enhanced by elevated N in stream water or by altered leaf chemistry. Instead, chronic experimental N enrichment shifted decomposition in streams from co-limitation to strong P limitation. Microbial respiration and extracellular enzyme production responded to altered nutrient availability in a manner consistent with resource allocation models. Divergent responses of terrestrial and aquatic decomposition to elevated N deposition likely arise from differences in water bioavailability. Our work highlights the value of simultaneously considering ecosystem processes in terrestrial and aquatic systems to understand the consequences of integrated landscape processes operating on large spatial scales.

     
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  4. null (Ed.)
    This paper considers system identification for systems whose output is asymptotically periodic under constant inputs. The model used for system identification is a discretetime Lur’e model consisting of asymptotically stable linear dynamics, a time delay, a washout filter, and a static nonlinear feedback mapping. For sufficiently large scaling of the loop transfer function, these components cause divergence under small signal levels and decay under large signal amplitudes, thus producing an asymptotically oscillatory output. A leastsquares technique is used to estimate the coefficients of the linear model as well as the parameters of a piecewise-linear approximation of the feedback mapping. 
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  5. null (Ed.)
  6. Free, publicly-accessible full text available January 22, 2025
  7. Self-excited systems arise in many applications, such as biochemical systems, mechanical systems with fluidstructure interaction, and fuel-driven systems with combustion dynamics. This paper presents a Lur’e model that exhibits biased oscillations under constant inputs. The model involves arbitrary asymptotically stable linear dynamics, time delay, a washout filter, and a saturation nonlinearity. For all sufficiently large scalings of the loop transfer function, these components cause divergence under small signal levels and decay under large signal amplitudes, thus producing an oscillatory response. A bias-generation mechanism is used to specify the mean of the oscillation. The main contribution of the paper is the presentation and analysis of a discrete-time version of this model. 
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  8. In some applications of control, the objective is to optimize the constant asymptotic response of the system by moving the state of the system from one forced equilibrium to another. Since suppression of the transient response is not the main objective, the feedback control law can operate quasistatically, that is, extremely slowly relative to the open-loop dynamics. Although integral control can be used to achieve the desired setpoint, three issues must be addressed, namely, nonlinearity, uncertainty, and multistability, where multistability refers to the fact that multiple locally stable equilibria may exist for the same constant input. In fact, multistability is the mechanism underlying hysteresis. The present paper applies an adaptive digital PID controller to achieve quasi-static control of systems that are nonlinear, uncertain, and multistable. The approach is demonstrated on multistable systems involving unmodeled cubic and backlash nonlinearities. 
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  9. null (Ed.)