Top-down processing of noisy and adversarial stimuli in deep generative models

This thesis studies the effect of adding a term usually neglected during the training phase of energy-based models. It is called ``KL term'' because of its dependence on Kullback-Leibler divergence, and it does not have a significant impact on the training in terms of running time and computational cost. I will initially present an analysis of its impact on training stability and some considerations relative to the general structure of the learning model. I will then study the denoising capabilities of the model by implementing top-down processes applied to different types of noisy input. Thirdly, to understand the quality of internal representations emerging in the hidden layers, I will apply a read-out classifier to the deepest hidden layer, calculating psychometric curves produced with different noise values. The final analysis will explore the model capability to resist to adversarial attacks using forward-backward iterations, also considering the spontaneous generative activity of the network by stimulating the read-out neurons relative to a specific class. Interestingly, results of this latter investigation are very encouraging for the MNIST dataset, suggesting that this type of energy-based models has potential to improve current defenses on adversarial attacks. However, for Cifar10 dataset it seems that more powerful computing hardware is needed to train models of larger sizes

Caratterizzazione delle sfere metriche nello spazio iperbolico complesso

Lo scopo di questo lavoro è dimostrare un teorema inedito nella letteratura, che fornisce una caratterizzazione delle superfici sferiche nello spazio iperbolico complesso in analogia all'equazione $A-N(A)=A_{0}$ per la sfera unitaria immersa in $\mathbb{R}^{n}$ di centro $A_{0}$ e direzione normale $N(A)$

Study of RF generators based on solid-state amplifiers for ion sources of Neutral Beam Injectors

Il lavoro di tesi svolto è incentrato sul sistema di alimentazione a radiofrequenza (RF PS) di SPIDER e mira a proporre un design concettuale basato sulla tecnologia dell'amplificatore RF allo stato solido. La tesi presenta inoltre lo studio dell'integrazione della soluzione allo stato solido nello specifico sistema di alimentazione di SPIDER (SPIDER PS), discutendo il merito di 2 soluzioni circuitali identificate e confrontandole dal punto di vista delle correnti di modo comune RF generate

An overview on B V Functions and Sets of Finite Perimeter in Rn

The aim of the thesis is to carry out an analysis on two topics: functions of bounded variation (which are L^1 functions defined on an open set of R^n whose distributional derivative is a Radon measure), and sets of finite perimeter in R^n (L^n measurable sets whose characteristic function belongs to the space B V). After considering some measure theory preliminary results, B V functions are defined and studied through the notion of variation, then the exposition narrows down just to B V functions of one variable and their properties. Finally, sets of finite perimeter are considered, together with related concepts such as Coarea Formula for B V functions, Isoperimetric Inequalities and the reduced boundary of a set