Aphids, Ants and Ladybirds: a mathematical model predicting their population dynamics

The interaction between aphids, ants and ladybirds has been investigated from an ecological point of view since many decades, while there are no attempts to describe it from a mathematical point of view. This paper introduces a new mathematical model to describe the within-season population dynamics in an ecological patch of a system composed by aphids, ants and ladybirds, through a set of four differential equations. The proposed model is based on the Kindlmann and Dixon set of differential equations, focused on the prediction of the aphids-ladybirds population densities, that share a prey-predator relationship. The population of ants, in mutualistic relationship with aphids and in interspecific competition with ladybirds, is described according to the Holland and De Angelis mathematical model, in which the authors faced the problem of mutualistic interactions in general terms. The set of differential equations proposed here is discretized by means the Nonstandard Finite Difference scheme, successfully applied by Gabbriellini to the mutualistic model. The constructed finite-difference scheme is positivity-preserving and characterized by four nonhyperbolic steady-states, as highlighted by the phase-space and time-series analyses. Particular attention is dedicated to the steady-state most interesting from an ecological point of view, whose asymptotic stability is demonstrated via the Centre Manifold Theory. The model allows to numerically confirm that mutualistic relationship effectively influences the population dynamic, by increasing the peaks of the aphids and ants population densities. Nonetheless, it is showed that the asymptotical populations of aphids and ladybirds collapse for any initial condition, unlike that of ants that, after the peak, settle on a constant asymptotic value

Thermal Control of a Dual Mode Parametric Sapphire Transducer

We propose a method to control the thermal stability of a sapphire dielectric transducer made with two dielectric disks separated by a thin gap and resonating in the whispering gallery (WG) modes of the electromagnetic field. The simultaneous measurement of the frequencies of both a WGH mode and a WGE mode allows one to discriminate the frequency shifts due to gap variations from those due to temperature instability. A simple model, valid in quasi equilibrium conditions, describes the frequency shift of the two modes in terms of four tuning parameters. A procedure for the direct measurement of them is presented.Comment: 5 pages, 6 figures, presented at EFTF-IFCS joint conference 200

Studio e caratterizzazione di un gradiometro gravitazionale per applicazioni geofisiche

Questo lavoro di tesi affronta lo studio e la caratterizzazione di un gradiometro gravitazionale sensibile alla componente diagonale gamma-zz del tensore gradiente di gravità. La misura di gradiente gravitazionale trova largo impiego nella prospezione geofisica per il riconoscimento di contrasti di densità prodotti da riserve di idrocarburi o da rilevanti discontinuità geologiche come ad esempio cavità sotterranee. Le accurate mappe di densità che la gradiometria gravimetrica consente di ottenere, trovano impiego, inoltre, nell’analisi di dati sismici. Per mettere a confronto la misura di gravità con la misura di gradiente di gravità ed evidenziare i benefici che quest’ultima introduce, vengono proposte delle simulazioni numeriche. Da queste si deduce che la gradiometria gravimetrica individua più facilmente della gravimetria, le anomalie prodotte da corpi geologici relativamente superficiali. Il gradiometro in fase di progettazione, misurerà la differenza di gravità tra due sensori accelerometrici separati da una distanza di 50 cm. Ciascun accelerometro è composto da un oscillatore meccanico accoppiato ad un sensore di spostamento costituito da un trasduttore posizione-frequenza. Il trasduttore consiste in una cavità formata da due dischi di zaffiro coassiali, risonante nei modi Whispering Gallery del campo elettromagnetico, la cui frequenza dipende dalla distanza tra tali dischi e quindi dalla posizione della massa di prova dell’oscillatore. La differenza tra le frequenze delle due cavità (e quindi tra le posizioni dei due accelerometri), è letta tramite un circuito interferometrico a microonde. Lo strumento ultimato raggiungerà la sensibilità di 3 Eotvos/Hz^(1/2) in un intervallo di frequenze tra 0.5-5 Hz e sarà caratterizzato da peso e dimensioni ridotte per renderlo facilmente trasportabile. Questo gradiometro funziona come strumento di zero. Ciò significa che è necessario un circuito di asservimento in grado di mantenere costantemente alla stessa frequenza di risonanza le due cavità durante l’esecuzione della misura. Questo avviene tramite un sistema di attuazione che consente di correggere la posizione dell’oscillatore con un segnale di correzione generato dal circuito di asservimento. Tale segnale di correzione, opportunamente calibrato, costituisce la misura del gradiente gravitazionale. Una parte del lavoro svolto in questa tesi riguarda la realizzazione del circuito di asservimento. Inoltre, poiché l’oscillazione degli accelerometri alla loro frequenza naturale produce un segnale differenziale all’uscita dell’interferometro che può portare fuori dalla dinamica, è stato ideato un circuito di controllo per smorzare tale segnale. Le prove sperimentali eseguite sull’apparato sperimentale, hanno permesso una prima valutazione del comportamento in temperatura ed una stima del CMRR (rapporto di reiezione di modo comune, del rumore sismico). Il limite fondamentale alla sensibilità dello strumento dipende dal rumore termico (o rumore di Nyquist) degli oscillatori meccanici. Poiché questa sorgente di rumore dipende inversamente dal fattore di merito meccanico Q, è necessario che gli accelerometri abbiano tale parametro il più elevato possibile. In questa tesi viene approfondito questo aspetto, mettendo in evidenza le cause che determinano un degrado del Q meccanico: in particolare viene studiato dal punto di vista teorico e sperimentale l’effetto di smorzmento viscoso provocato dall’aria compresa tra i dischi di zaffiro. Saranno inoltre trattati gli effetti della pressione dell’aria sul Q meccanico

Deep learning to estimate the basement depth by gravity data using a feedforward neural network

We have developed a deep-learning method based on the neural network of the feedforward type to estimate the depth to the basement from potential fields. The data used to train and test the network are related to the Bishop synthetic model. A trial-and-error approach was used to find the hyperparameters that have the best compromise between performance and computation time. The training was organized by associating the depth values of the basement to the data through a moving window, running along profiles in the north–south and east–west directions. In this way, we generated a set of approximately 296,980 examples. We verified the robustness of the trained net by carrying out a test related to another syn- thetic model, extracted from the Himalaya digital elevation model. The inherent ambiguity of the problem led us to test two hypotheses for the estimation of the basement depth, the first related to a priori information on the density contrast and the shallowest depth and the second assuming the knowledge of the depth at least at two points, but not that of the density contrast. In these cases, our data-driven approach yielded interesting results leading to estimate the maximum depth in the first case and the density contrast in the second one. We finally applied the method to the isostatic anomaly of the Yucca Flat sedimentary basin, Nevada. The results are consistent with previous interpretations of the area, which were based on gravity inversion methods

Complex Contagions and the Diffusion of Innovations: Evidence from a Small-N Study

International audienceThe recent literature on "complex contagions" challenges Granovetter's classic hypothesis on the strength of weak ties and argues that, when the actors' choice requires reinforcement from several sources, it is the structure of strong ties that really matters to sustain rapid and wide diffusion. The paper contributes to this debate by reporting on a small-N study that relies on a unique combination of ethnographic data, social network analysis, and computational models. In particular, we investigate two rural populations of Indian and Kenyan potters who have to decide whether adopting new, objectively more efficient and economically more attractive, technical/stylistic options. Qualitative field data show that religious sub-communities within the Indian and Kenyan populations exhibit markedly different diffusion rates and speed over the last thirty years. To account for these differences, we first analyze empirically observed kinship networks and advice networks, and, then, we recreate the actual aggregate diffusion curves through a series of empirically-calibrated agent-based simulations. Combining the two methods, we show that, while single exposure through heterophilious weak ties were sufficient to initiate the diffusion process, large bridges made of strong ties can in fact lead to faster or slower diffusion depending on the type of signals circulating in the network. We conclude that, even in presence of "complex contagions", dense local ties cannot be regarded as a sufficient condition for faster diffusion

Introducing qualitative and social science factors in archaeological modelling: necessity and relevance.

International audienceDespite having been a pedigree stretching for several decades, computational approaches remain highly debated in archaeology, hailed by some as the future of the discipline, and discarded by some as a poor, arrogant and overgeneralizing attempt at mimicking the past. This introductory chapter argues that traditional criticisms made at computational models stem from several fundamental misunderstandings. In particular, several archaeologists favouring either a more "social theory" perspective or a more "fieldwork first and avoid generalizing" approach have negatively commented upon what they perceived as simple models when compared to the complex, holistic nature of social life. We argue here that modelling scientific teams, combining archaeologists and modellers, are aware of these complexity and uncertainty and rather prefer to tackle it by explicitly focusing upon a minimal set of epistemological procedures, concepts and parameters, set in an explicitly formal environment. The implications of this epistemological standpoint are evaluated in view of the various contributions to this volume, presented at the end of this contribution