Existence of discrete-time LQG-controllers

Existence results for the LQG-controller are investigated. An infimal Riccati equation based controller may potentially give closed loop eigenvalues on the unit circle. Assuming left and right invertibility it is shown that there exists an optimal controller if and only if the Riccati equation based controller stabilizes the closed loop system after removal of all its unobservable and uncontrollable modes. Furthermore this reduced controller is the optimal controller, and its transfer function is unique. This existence condition is a considerable simplification of the more general geometric condition recently derived by Trentelman and Stoorvogel.     

Renal sodium excretion after oral or intravenous sodium loading in sodium-deprived normotensive and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats that had been on a low sodium diet for 3 days were given 1.5 mmol sodium chloride kg-1 body weight either orally or intravenously. The rats receiving an oral sodium load showed a greater natriuresis than those receiving the same saline load intravenously. No increase of renal sodium excretion was observed when the rats received a hypertonic mannitol solution orally. The cumulative sodium excretion during the 8 h following oral loading was two to three times larger in SHR than in WKY, whereas no difference between strains could be demonstrated after giving saline intravenously. Furthermore, after switching from normal to low sodium diet the rate of decrease of renal sodium excretion was greater in SHR than in WKY rats. It is proposed that there exists a gastrointestinal sensory mechanism for sodium controlling the renal sodium excretion. Furthermore, it is suggested that the function of this mechanism differs between SHR and WKY.     

Probabilistic heuristic estimates

Though they constitute the major knowledge source in problem-solving systems, no unified theory of heuristics has emerged. Pearl [15] defines heuristics as criteria, methods, or principles for deciding which among several alternative courses of action promises to be the most effective in order to achieve some goal. The absence of a more precise definition has impeded our efforts to understand, utilize, and discover heuristics. Another consequence is that problem-solving techniques which rely on heuristic knowledge cannot be relied upon to act rationally — in the sense of the normative theory of rationality.To provide a sound basis for BPS, the Bayesian Problem-Solver, we have developed a simple formal theory of heuristics, which is general enough to subsume traditional heuristic functions as well as other forms of problem-solving knowledge, and to straddle disparate problem domains. Probabilistic heuristic estimates represent a probabilistic association of sensations with prior experience — specifically, a mapping from observations directly to subjective probabilities which enables the use of theoretically principled mechanisms for coherent inference and decision making during problem-solving. This paper discusses some of the implications of this theory, and describes its successful application in BPS.This research was made possible by support from Heuristicrats, the National Aeronautics and Space Administration, and the Rand Corporation.     

Pheromonal secretions from glands on the 5th abdominal sternite of hydropsychid and rhyacophilid caddisflies (Trichoptera)

Extracts of different body parts of adult Trichoptera were tested for electrophysiological activity. Extracts of the IVth and Vth abdominal sternites of femaleHydropsyche angustipennis, Rhyacophila nubila, andR. fasciata, containing a paired exocrine gland, elicited significant electroan-tennographic responses when tested on conspecific male antennae. The paired gland occurs also in males of all the species, and inH. angustipennis, extracts from males were more active than female extracts when tested on male antennae. Female and male extracts from all species were analyzed by gas chromatography with simultaneous flame ionization and electroantennographic detection (EAD). EAD-active peaks in female extracts, stimulating male antennae, were identified inH. angustipennis as nonan-2-one; and inR. nubila andR. fasciata as heptan-2-one, heptan-2-ol, nonan-2-one, and nonan-2-ol. EAD-active components from maleH. angustipennis stimulating male antennae were octan-2-one, nonan-2-one (major peak), (Z)-6-nonen-2-one, decan-2-one, and a methylbranched decan-2-one. Female extracts and synthetic mixtures of compounds identified from femaleH. angustipennis andR. fasciata were tested for attractivity in the field. High catches with control traps obscured the results, but a synthetic mixture of the four identified compounds was significantly attractive and not different from female extracts for attracting maleR. fasciata. InH. angustipennis, a synthetic six-component male blend, in which nonan-2-one was the major component, attracted significant numbers of male and femaleH. angustipennis. Extracts of maleR. nubila andR. fasciata contained acetophenone and hexanoic and octanoic acids but did not have any electrophysiological or behavioral activity on either male or female antennae of conspecifics. The occurrence of a female sex pheromone inRhyacophila and an aggregation pheromone inHydropsyche corresponds to earlier described differences in mating behaviors in the Rhyacophilidae and Hydropsychidae.     

High-Temperature Crack Growth in Monolithic and SiCw-Reinforced Silicon Nitride under Static and Cyclic Loads

Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si₃N₄ at 1450°C and a hot–pressed Si₃N₄–10 vol% SiC_w composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiC_w-reinforced Si₃N₄ are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si₃N₄–based ceramics is compared with that of oxide-based ceramics.     

Single-stage flowing liquid film impactor for continuous on-line particle analysis

A single-stage “wet impactor” is presented, where the impaction occurs on a regenerated water surface. The developed impactor is equipped with an impaction liquid support plate of etched glass and a drain spout providing a continuous liquid flow covering the impaction area. Subsequent transport of the impaction liquid makes an on-line determination possible. With multiple nozzles (74 holes, 0.3 mm i.d.) and an air flow of 101/min the cut-off was determined to 0.41 ± 0.02 μm. The impactor was also investigated for its particle loss. The cut-off function, regarding the consequences of letting impaction occur in a liquid film is discussed and compared to conventional impactors. The analysis technique was tested in an ambient air measurement study with an ion chromatograph attached to the sampling system.     

Pheromone specificity inEriocrania semipurpurella (Stephens) andE. sangii (Wood) (Lepidoptera: Eriocraniidae) based on chirality of semiochemicals

The fifth abdominal segment of femaleEriocrania semipurpurella (Stephens) andE. sangii (Wood) contains a pair of exocrine glands. Hexane extracts of this segment were prepared from both species and analyzed by gas chromatography with simultaneous flame ionization and electroantennographic detection (EAD). For both species, the EAD active peaks were identified as nonan-2-one, (Z)-6-nonen-2-one, and (Z)-6-nonen-2-ol by means of mass spectrometry and comparison of retention indices with those of synthetic standards. Enantiomeric separation of chiral alcohols from the female extracts was achieved by gas chromatographic analysis on a cyclodextrin column. InE. semipurpurella, a mixture of (2S,6Z)-nonen-2-ol and (2R,6Z)-nonen-2-ol (2: I) was found, whereas inE. sangii (2S,6Z)-nonen-2-ol was the predominant enantiomer and only traces of theR enantiomer were indicated by the antennal response. In field tests, a blend of the three compounds was not attractive to conspecific males. A subtractive assay showed that the alcohol in various enantiomeric mixtures was the only attractive compound, whereas addition of (Z)-6-nonen-2-one to the alcohol completely inhibited the attraction of both species. A trapping experiment including a wide range of ratios between theR andS enantiomers showed that baits containing 95–100% of theS enantiomer were attractive to maleE. sangii, whereas males ofE. semipurpurella were attracted to all tested ratios of the enantiomers. However, the response profiles of maleE. semipurpurella differed between populations from southern Sweden, south Finland, and the Kola Peninsula in Russia. In south Sweden males were maximally attracted to a racemic mixture of the alcohols. At the Kola PeninsulaE. semipurpurella was attracted to baits containing 95–100% of theR enantiomer. In south Finland all tested ratios between 0 and 100%R enantiomer trappedE. semipurpurella, but the trap catches appeared to be bimodally distributed with peaks around 15 and 70%R enantiomer. The trapping results suggest the existence of pheromone races or sibling species among the specimens identified asE. semipurpurella.Dedicated to Prof. H. J. Bestmann on the occasion of his 70th birthday.     

Host Plant Constancy in Ovipositing Manduca sexta

Journal of Chemical Ecology - Many pollinating insects exhibit flower constancy, i.e. they target flower species they have already experienced and fed from. While the insects might profit from...