In vivo visualization of abdominal malignancies with acoustic radiation force elastography.

The utility of acoustic radiation force impulse (ARFI) imaging for real-time visualization of abdominal malignancies was investigated. Nine patients presenting with suspicious masses in the liver (n = 7) or kidney (n = 2) underwent combined sonography/ARFI imaging. Images were acquired of a total of 12 tumors in the nine patients. In all cases, boundary definition in ARFI images was improved or equivalent to boundary definition in B-mode images. Displacement contrast in ARFI images was superior to echo contrast in B-mode images for each tumor. The mean contrast for suspected hepatocellular carcinomas (HCCs) in B-mode images was 2.9 dB (range: 1.5-4.2) versus 7.5 dB (range: 3.1-11.9) in ARFI images, with all HCCs appearing more compliant than regional cirrhotic liver parenchyma. The mean contrast for metastases in B-mode images was 3.1 dB (range: 1.2-5.2) versus 9.3 dB (range: 5.7-13.9) in ARFI images, with all masses appearing less compliant than regional non-cirrhotic liver parenchyma. ARFI image contrast (10.4 dB) was superior to B-mode contrast (0.9 dB) for a renal mass. To our knowledge, we present the first in vivo images of abdominal malignancies in humans acquired with the ARFI method or any other technique of imaging tissue elasticity.

Intracardiac acoustic radiation force impulse (ARFI) and shear wave imaging in pigs with focal infarctions.

Four pigs, three with focal infarctions in the apical intraventricular septum (IVS) and/or left ventricular free wall (LVFW), were imaged with an intracardiac echocardiography (ICE) transducer. Custom beam sequences were used to excite the myocardium with focused acoustic radiation force (ARF) impulses and image the subsequent tissue response. Tissue displacement in response to the ARF excitation was calculated with a phase-based estimator, and transverse wave magnitude and velocity were each estimated at every depth. The excitation sequence was repeated rapidly, either in the same location to generate 40 Hz M-modes at a single steering angle, or with a modulated steering angle to synthesize 2-D displacement magnitude and shear wave velocity images at 17 points in the cardiac cycle. Both types of images were acquired from various views in the right and left ventricles, in and out of infarcted regions. In all animals, acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) estimates indicated diastolic relaxation and systolic contraction in noninfarcted tissues. The M-mode sequences showed high beat-to-beat spatio-temporal repeatability of the measurements for each imaging plane. In views of noninfarcted tissue in the diseased animals, no significant elastic remodeling was indicated when compared with the control. Where available, views of infarcted tissue were compared with similar views from the control animal. In views of the LVFW, the infarcted tissue presented as stiff and non-contractile compared with the control. In a view of the IVS, no significant difference was seen between infarcted and healthy tissue, whereas in another view, a heterogeneous infarction was seen to be presenting itself as non-contractile in systole.

The interference effects of non-rotated versus counter-rotated trials in visuomotor adaptation

An isometric torque-production task was used to investigate interference and retention in adaptation to multiple visuomotor environments. Subjects produced isometric flexion-extension and pronation-supination elbow torques to move a cursor to acquire targets as quickly as possible. Adaptation to a 30 degrees counter-clockwise (CCW) rotation (task A), was followed by a period of rest (control), trials with no rotation (task B0), or trials with a 60 degrees clockwise (CW) rotation (task B60). For all groups, retention of task A was assessed 5 h later. With initial training, all groups reduced the angular deviation of cursor paths early in the movements, indicating feedforward adaptation. For the control group, performance at commencement of the retest was significantly better than that at the beginning of the initial learning. For the B0 group, performance in the retest of task A was not dissimilar to that at the start of the initial learning, while for the B60 group retest performance in task A was markedly worse than initially observed. Our results indicate that close juxtaposition of two visuomotor environments precludes improved retest performance in the initial environment. Data for the B60 group, specifically larger angular errors upon retest compared with initial exposures, are consistent with the presence of anterograde interference. Furthermore, full interference occurred even when the visuomotor environment encountered in the second task was not rotated (B0). This latter novel result differs from those obtained for force field learning, where interference does not occur when task B does not impose perturbing forces, i.e., when B consists of a null field (Brashers-Krug et al., Nature 382:252-255, 1996). The results are consistent with recent proposals suggesting different interference mechanisms for visuomotor (kinematic) compared to force field (dynamic) adaptations, and have implications for the use of washout trials when studying interference between multiple visuomotor environments.

Quasi-classical model of non-destructive wavepacket manipulation by intense ultrashort nonresonant laser pulses

A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 10(13) W cm(-2) are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typical of coherent control and femtochemistry. The 1s sigma ground state of the D-2 precursor is projected onto the available electronic states in D-2(+) (1s sigma(g) ground and 2p sigma(u) dissociative) and D+ + D+ (Coulomb explosion) by tunnel ionization by an ultrashort 'pump' pulse, and relative populations are found numerically. A generalized non-adiabatic treatment allows the dependence of the initial vibrational population distribution on laser intensity to be calculated. The wavepacket is approximated as a classical ensemble of particles moving on the 1s sigma(g) potential energy surface (PES), and hence follow trajectories of different amplitudes and frequencies depending on the initial vibrational state. The 'control' pulse introduces a time-dependent polarization of the molecular orbital, causing the PES to be modified according to the dynamic Stark effect and the transition dipole. The trajectories adjust in amplitude, frequency and phase-offset as work is done on or by the resulting force; comparing the perturbed and unperturbed trajectories allows the final vibrational state populations and phases to be determined. The action of the 'probe' pulse is represented by a discrete internuclear boundary, such that elements of the ensemble at a larger internuclear separation are assumed to be photodissociated. The vibrational populations predicted by the QCM are compared to recent quantum simulations (Niederhausen and Thumm 2008 Phys. Rev. A 77 013404), and a remarkable agreement has been found. The applicability of this model to femtosecond and attosecond time-scale experiments is discussed and the relation to established femtochemistry and coherent control techniques are explored.

Dispersion interactions in room-temperature ionic liquids: Results from a non-empirical density functional

The role of dispersion or van de Waals (VDW) interactions in imidazolium-based room-temperature ionic liquids is studied within the framework of density functional theory, using a recently developed non-empirical functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)], as efficiently implemented in the SIESTA code [G. Roman-Perez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009)]. We present results for the equilibrium structure and lattice parameters of several crystalline phases, finding a general improvement with respect to both the local density (LDA) and the generalized gradient approximations (GGA). Similar to other systems characterized by VDW bonding, such as rare gas and benzene dimers as well as solid argon, equilibrium distances and volumes are consistently overestimated by approximate to 7%, compared to -11% within LDA and 11% within GGA. The intramolecular geometries are retained, while the intermolecular distances and orientations are significantly improved relative to LDA and GGA. The quality is superior to that achieved with tailor-made empirical VDW corrections ad hoc [M. G. Del Popolo, C. Pinilla, and P. Ballone, J. Chem. Phys. 126, 144705 (2007)]. We also analyse the performance of an optimized version of this non-empirical functional, where the screening properties of the exchange have been tuned to reproduce high-level quantum chemical calculations [J. Klimes, D. Bowler, and A. Michaelides, J. Phys.: Condens. Matter 22, 074203 (2010)]. The results for solids are even better with volumes and geometries reproduced within 2% of experimental data. We provide some insight into the issue of polymorphism of [bmim][Cl] crystals, and we present results for the geometry and energetics of [bmim][Tf] and [mmim][Cl] neutral and charged clusters, which validate the use of empirical force fields. (C) 2011 American Institute of Physics. [doi:10.1063/1.3652897]

Statistical-mechanical description of classical test-particle dynamics in the presence of an external force field: modelling noise and damping from first principles

Aiming to establish a rigorous link between macroscopic random motion (described e.g. by Langevin-type theories) and microscopic dynamics, we have undertaken a kinetic-theoretical study of the dynamics of a classical test-particle weakly coupled to a large heat-bath in thermal equilibrium. Both subsystems are subject to an external force field. From the (time-non-local) generalized master equation a Fokker-Planck-type equation follows as a "quasi-Markovian" approximation. The kinetic operator thus defined is shown to be ill-defined; in specific, it does not preserve the positivity of the test-particle distribution function f(x, v; t). Adopting an alternative approach, previously introduced for quantum open systems, is proposed to lead to a correct kinetic operator, which yields all the expected properties. A set of explicit expressions for the diffusion and drift coefficients are obtained, allowing for modelling macroscopic diffusion and dynamical friction phenomena, in terms of an external field and intrinsic physical parameters.

Force-induced activation of covalent bonds in mechanoresponsive polymeric materials

Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and
bone1–6. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure7. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress
favourably altersmaterial properties. A few mechanosensitive polymers with this property have been developed8–14; but their active response is mediated through non-covalent processes, which may
limit the extent to which properties can be modified and the longterm stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups—so-called mechanophores—that the directional nature of mechanical forces can selectively break and re-form covalent bonds15,16. We now demonstrate that such forceinduced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.

Resolution theory, and static and frequency-dependent cross-talk in piezoresponse force microscopy

Probing the functionality of materials locally by means of scanning probe microscopy (SPM) requires a reliable framework for identifying the target signal and separating it from the effects of surface morphology and instrument non-idealities, e.g. instrumental and topographical cross-talk. Here we develop a linear resolution theory framework in order to describe the cross-talk effects, and apply it for elucidation of frequency-dependent cross-talk mechanisms in piezoresponse force microscopy. The use of a band excitation method allows electromechanical/electrical and mechanical/topographic signals to be unambiguously separated. The applicability of a functional fit approach and multivariate statistical analysis methods for identification of data in band excitation SPM is explored.

Discursive Control, Non-Domination and Hegelian Recognition Theory: Marrying Pettit’s Account(s) of Freedom with a Pippinian/Brandomian Reading of Hegelian Agency

The aim of this article is to combine Pettit’s account(s) of freedom, both his work on discursive control and on non-domination, with Pippin’s and Brandom’s reinterpretation of Hegelian rational agency and the role of recognition theory within it. The benefits of combining these two theories lie, as the article hopes to show, in three findings: first, re-examining Hegelian agency in the spirit of Brandom and Pippin in combination with Pettit’s views on freedom shows clearly why and in which way a Hegelian account of rational agency can ground an attractive socio-political account of freedom; second, the reconciling of discursive control and non-domination with Hegelian agency shows how the force and scope of recognition become finally tangible, without either falling into the trap of overburdening the concept, or merely reducing it to the idea of simple respect; third, the arguments from this article also highlight the importance of freedom as non-domination and how this notion is, indeed, as Pettit himself claims, an agency-freedom which aims at successfully securing the social, political, economic and even (some) psychological conditions for free and autonomous agency.

Piezoresponse force microscopy of ferroelectric relaxors

Nesta tese, ferroeléctricos relaxor (I dont know uf the order is correct) de base Pb das familias (Pb,La)(Zr,Ti)O3 (PLZT), Pb(Mg1/3,Nb2/3)O3-PbTiO3 (PMN-PT), Pb(Zn1/3,Nb2/3)O3-PbTiO3 (PZN-PT) foram investigados e analisados. As propriedades ferroeléctricas e dieléctricas das amostras foram estudadas por métodos convencionais de macro e localmente por microscopia de força piezoeléctrica (PFM). Nos cerâmicos PLZT 9.75/65/35 o contraste da PFM à escala nanometrica _ foi investigado em função do tamanho e orientação dos grãos. Apurou-se que a intensidade do sinal piezoeléctrico das nanoestruturas diminui com o aumento da temperatura e desaparece a 490 K (La mol. 8%) e 420 K (9,5%). Os ciclos de histerese locais foram obtidos em função da temperatura. A evolução dos parâmetros macroscópicos e locais com a temperatura de superfície sugere um forte efeito de superfície nas transições de fase ferroeléctricas do material investigado. A rugosidade da parede de domínio é determinada por PFM para a estrutura de domínio natural existente neste ferroeléctrico policristalino. Além disso, os domínios ferroeléctricos artificiais foram criados pela aplicação de pulsos eléctricos à ponta do condutor PFM e o tamanho de domínio in-plane foi medido em função da duração do pulso. Todas estas experiências levaram à conclusão de que a parede de domínio em relaxors do tipo PZT é quase uma interface unidimensional. O mecanismo de contraste na superfície de relaxors do tipo PLZT é medido por PFMAs estruturas de domínio versus evolução da profundidade foram estudadas em cristais PZN-4,5%PT, com diferentes orientações através da PFM. Padrões de domínio irregulares com tamanhos típicos de 20-100 nm foram observados nas superfícies com orientação <001> das amostras unpoled?. Pelo contrário, os cortes de cristal <111> exibem domínios regulares de tamanho mícron normal, com os limites do domínio orientados ao longo dos planos cristalográficos permitidos. A existência de nanodomínios em cristais com orientação <001> está provisoriamente (wrong Word) atribuída à natureza relaxor de PZN-PT, onde pequenos grupos polares podem formar-se em coindições de zero-field-cooling (ZFC). Estes nanodomínios são considerados como os núcleos do estado de polarização oposta e podem ser responsáveis pelo menor campo coercitivo para este corte de cristal em particular. No entanto, a histerese local piezoelétrica realizada pelo PFM à escala nanométrica indica uma mudança de comportamento de PZN-PT semelhante para ambas as orientações cristalográficas investigadas. A evolução das estruturas de domínio com polimento abaixo da superfície do cristal foi investigada. O domínio de ramificações e os efeitos de polarização de triagem após o polimento e as medições de temperatura têm sido estudados pela PFM e pela análise SEM. Além disso, verificou-se que a intensidade do sinal piezoeléctrico a partir das estruturas de nanodomínio diminui com o aumento da temperatura, acabando por desaparecer aos 430 K (orientaçáo <111>) e 470 K (orientação <100>). Esta diferença de temperatura nas transições de fase local em cristais de diferentes orientações é explicada pelo forte efeito de superfície na transição da fase ferroelétrica em relaxors.A comutação da polarização em relaxor ergódico e nas fases ferroeléctricas do sistema PMN-PT foram realizadas pela combinação de três métodos, Microscopia de Força Piezoeléctrica, medição de um único ponto de relaxamento eletromecânico e por ultimo mapeamento de espectroscopia de tensão. A dependência do comportamento do relaxamento na amplitude e tempo da tensão de pulso foi encontrada para seguir um comportamento logarítmico universal com uma inclinação quase constante. Este comportamento é indicativo da progressiva população dos estados de relaxamento lento, ao contrário de uma relaxação linear na presença de uma ampla distribuição do tempo de relaxamento. O papel do comportamento de relaxamento, da não-linearidade ferroeléctrica e da heterogeneidade espacial do campo na ponta da sonda de AFM sobre o comportamento do ciclo de histerese é analisada em detalhe. Os ciclos de histerese para ergódica PMN- 10%PT são mostrados como cineticamente limitados, enquanto que no PMN, com maior teor de PT, são observados verdadeiros ciclos de histerese ferroeléctrica com viés de baixa nucleação.

Le fonctionnement du régime de preuve libre dans un système non-expert : le traitement symptomatique des preuves par la Section de la protection des réfugiés

Un résumé en anglais est également disponible.

Étude qualitative de la force mentale menée auprès d’athlètes québécois francophones de niveau universitaire

L’objectif de cette étude était d’obtenir une meilleure compréhension du concept de force mentale appliqué dans un contexte sportif. Pour se faire, des entrevues ont été réalisées avec dix athlètes québécois francophones de niveau universitaire. L’analyse des données s’est effectuée en suivant les principes de la version abrégée de la théorisation ancrée. Les résultats appuient globalement la littérature scientifique. En somme, la force mentale serait composée de caractéristiques de base interreliées, telles que la motivation internalisée, la confiance en soi, la maturité et la détermination. La force mentale serait sollicitée non seulement lorsque l’athlète fait face à des situations difficiles comme l’adversité, mais également lorsque le contexte dans lequel il évolue est exempt de complications (à l’entraînement ou lors de victoire). Les résultats des entrevues indiquent que face à ces deux types de situation, les athlètes forts mentalement possèderaient un mode réactionnel tridimensionnel (dimension cognitive, affective et comportementale) se caractérisant par la gestion et le contrôle de l’attention, l’interprétation optimiste des situations, le contrôle de l’activation et du langage corporel, la gestion des émotions et la résilience. Contrairement à la littérature, les résultats suggèrent que la force mentale ne serait pas nécessairement liée aux athlètes performants au plus haut niveau de leur sport, mais à la capacité des athlètes à donner de bonnes performances en fonction de leur potentiel individuel. Cette étude constitue le premier pas vers le développement d’un instrument de mesure permettant d’évaluer le niveau de force mentale chez les athlètes francophones.

Comparaison de la force d'adhésion de deux types de résines de collage appliquées sur des surfaces métalliques traitées de différentes façons : une étude in vitro

L’objectif de cette étude était d’évaluer et de comparer la force d’adhésion de deux résines de collage: le ClearfilTM Esthetic Cement & DC Bond Kit (C) et le RelyXTM Unicem (R), sur trois adhérents différents : une surface d’émail, un alliage de métaux non précieux (Np) et un alliage de métaux semi-précieux (Sp). La surface des échantillons des alliages métalliques a subi différents traitements de surface. Sur l’émail (n=15) ainsi que sur les plaquettes d’alliages Np et Sp (n=15), des cylindres de résine étaient appliqués et polymérisés. Suite au processus de collage, les échantillons ont été incubés à 37°C pendant 24 heures, puis ont subi 500 cycles de thermocyclage. Des tests de cisaillement ont été effectués, suivi par l’analyse de la surface des échantillons au microscope à balayage électronique. Une comparaison de type T-test et des comparaisons multiples post hoc, ont été effectuées pour l’analyse statistique (p 0,05). Sur l’émail, les résultats ont démontré que la résine C présentait une force d'adhésion moyenne statistiquement supérieure (33,97±17,18 MPa) à la résine R (10,48±11,23 MPa) (p 0,05). Le type d’alliage utilisé n’influençait pas la force d’adhésion, et ce, peu importe le type de résine de collage (p>0,05). Pour le groupe Sp, la résine C a démontré une adhésion statistiquement supérieure à la résine R, et ce, pour tous les traitements de surface (p 0,05). En conclusion, la résine C a démontré des résultats d’adhésion significativement supérieurs à la résine R sur l’émail ainsi que sur presque toutes les surfaces traitées des alliages de métaux.

L'élastographie ultrasonore dynamique vasculaire : une nouvelle modalité d'imagerie non-invasive pour la caractérisation mécanique de la thrombose veineuse

L’accident thromboembolique veineux, tel que la thrombose veineuse profonde (TVP) ou thrombophlébite des membres inférieurs, est une pathologie vasculaire caractérisée par la formation d’un caillot sanguin causant une obstruction partielle ou totale de la lumière sanguine. Les embolies pulmonaires sont une complication mortelle des TVP qui surviennent lorsque le caillot se détache, circule dans le sang et produit une obstruction de la ramification artérielle irriguant les poumons. La combinaison d’outils et de techniques d’imagerie cliniques tels que les règles de prédiction cliniques (signes et symptômes) et les tests sanguins (D-dimères) complémentés par un examen ultrasonographique veineux (test de compression, écho-Doppler), permet de diagnostiquer les premiers épisodes de TVP. Cependant, la performance de ces outils diagnostiques reste très faible pour la détection de TVP récurrentes. Afin de diriger le patient vers une thérapie optimale, la problématique n’est plus basée sur la détection de la thrombose mais plutôt sur l’évaluation de la maturité et de l’âge du thrombus, paramètres qui sont directement corrélées à ses propriétés mécaniques (e.g. élasticité, viscosité). L’élastographie dynamique (ED) a récemment été proposée comme une nouvelle modalité d’imagerie non-invasive capable de caractériser quantitativement les propriétés mécaniques de tissus. L’ED est basée sur l’analyse des paramètres acoustiques (i.e. vitesse, atténuation, pattern de distribution) d’ondes de cisaillement basses fréquences (10-7000 Hz) se propageant dans le milieu sondé. Ces ondes de cisaillement générées par vibration externe, ou par source interne à l’aide de la focalisation de faisceaux ultrasonores (force de radiation), sont mesurées par imagerie ultrasonore ultra-rapide ou par résonance magnétique. Une méthode basée sur l’ED adaptée à la caractérisation mécanique de thromboses veineuses permettrait de quantifier la sévérité de cette pathologie à des fins d’amélioration diagnostique. Cette thèse présente un ensemble de travaux reliés au développement et à la validation complète et rigoureuse d’une nouvelle technique d’imagerie non-invasive élastographique pour la mesure quantitative des propriétés mécaniques de thromboses veineuses. L’atteinte de cet objectif principal nécessite une première étape visant à améliorer les connaissances sur le comportement mécanique du caillot sanguin (sang coagulé) soumis à une sollicitation dynamique telle qu’en ED. Les modules de conservation (comportement élastique, G’) et de perte (comportement visqueux, G’’) en cisaillement de caillots sanguins porcins sont mesurés par ED lors de la cascade de coagulation (à 70 Hz), et après coagulation complète (entre 50 Hz et 160 Hz). Ces résultats constituent les toutes premières mesures du comportement dynamique de caillots sanguins dans une gamme fréquentielle aussi étendue. L’étape subséquente consiste à mettre en place un instrument innovant de référence (« gold standard »), appelé RheoSpectris, dédié à la mesure de la viscoélasticité hyper-fréquence (entre 10 Hz et 1000 Hz) des matériaux et biomatériaux. Cet outil est indispensable pour valider et calibrer toute nouvelle technique d’élastographie dynamique. Une étude comparative entre RheoSpectris et la rhéométrie classique est réalisée afin de valider des mesures faites sur différents matériaux (silicone, thermoplastique, biomatériaux, gel). L’excellente concordance entre les deux technologies permet de conclure que RheoSpectris est un instrument fiable pour la mesure mécanique à des fréquences difficilement accessibles par les outils actuels. Les bases théoriques d’une nouvelle modalité d’imagerie élastographique, nommée SWIRE (« shear wave induced resonance dynamic elastography »), sont présentées et validées sur des fantômes vasculaires. Cette approche permet de caractériser les propriétés mécaniques d’une inclusion confinée (e.g. caillot sanguin) à partir de sa résonance (amplification du déplacement) produite par la propagation d’ondes de cisaillement judicieusement orientées. SWIRE a également l’avantage d’amplifier l’amplitude de vibration à l’intérieur de l’hétérogénéité afin de faciliter sa détection et sa segmentation. Finalement, la méthode DVT-SWIRE (« Deep venous thrombosis – SWIRE ») est adaptée à la caractérisation de l’élasticité quantitative de thromboses veineuses pour une utilisation en clinique. Cette méthode exploite la première fréquence de résonance mesurée dans la thrombose lors de la propagation d’ondes de cisaillement planes (vibration d’une plaque externe) ou cylindriques (simulation de la force de radiation par génération supersonique). DVT-SWIRE est appliquée sur des fantômes simulant une TVP et les résultats sont comparés à ceux donnés par l’instrument de référence RheoSpectris. Cette méthode est également utilisée avec succès dans une étude ex vivo pour l’évaluation de l’élasticité de thromboses porcines explantées après avoir été induites in vivo par chirurgie.

Le paradoxe dans les Alices de Lewis Carroll : la force du littéraire dans la théorisation de l'irrésoluble /

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal