Dans cette thèse, nous étudions les problèmes de la factorisation d'entier et de calcul de logarithme discret dans les corps finis. Dans un premier temps, nous nous intéressons à l'algorithme de factorisation d'entier ECM et présentons une méthode pour analyser les courbes elliptiques utilisées dans cet algorithme en étudiant les propriétés galoisiennes des polynômes de division. Ensuite, nous présentons en détail l'algorithme de factorisation d'entier NFS, et nous nous intéressons en particulier à l'étape de sélection polynomiale pour laquelle des améliorations d'algorithmes existants sont proposées. Puis, nous présentons les algorithmes NFS-DL et FFS pour le calcul de logarithme discret dans les corps finis. Nous donnons aussi des détails sur deux calculs de logarithme discret effectués durant cette thèse, l'un avec NFS-DL et l'autre avec FFS. Enfin, nous étudions une étape commune à l'algorithme NFS pour la factorisation et aux algorithmes NFS-DL et FFS pour le calcul de logarithme discret: l'étape de filtrage. Nous l'étudions en détail et nous présentons une amélioration dont nous validons l'impact en utilisant des données provenant de plusieurs calculs de factorisation et de logarithme discret / In this thesis, we study the problems of integer factorization and discrete logarithm computation in finite fields. First, we study the ECM algorithm for integer factorization and present a method to analyze the elliptic curves used in this algorithm by studying the Galois properties of division polynomials. Then, we present in detail the NFS algorithm for integer factorization and we study in particular the polynomial selection step for which we propose improvements of existing algorithms. Next, we present two algorithms for computing discrete logarithms in finite fields: NFS-DL and FFS. We also give some details of two computations of discrete logarithms carried out during this thesis, one with NFS-DL and the other with FFS. Finally, we study a common step of the NFS algorithm for integer factorization and the NFS-DL and FFS algorithms for discrete logarithm computations: the filtering step. We study this step thoroughly and present an improvement for which we study the impact using data from several computations of discrete logarithms and factorizations

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Ricefield based fish seed production (RBFSP) in irrigated spring (boro) ricefields after initial introductions by external promoters has spread among farmers in parts of Northwest Bangladesh. This approach to producing juvenile fish, rather than by specialised geographically clustered hatchery and nursery enterprises, has been recognised as a strategy for decentralised production that makes large high quality seed available locally and supports food fish production. RBFSP has been promoted by the international NGO CARE as part of a process to improve rice-based livelihoods of farming households using a farmer field school (FFS) approach in two consecutive projects between 1993 and 2005. The approach is technically simple and is based on the stocking of common carp (Cyprinus carpio) eggs and Nile tilapia (Oreochromis niloticus) broodfish in ricefields. As a new approach to farmer level fish seed production, its livelihood impacts on the farming households as well as associated actors; its adoption, adaptation and rejection process in farming households; and its cost-effectives for dissemination at farmer level were not well understood. This thesis mainly applies the concept of the sustainable livelihood approach (SLA) using tools and processes of the growing family of participatory research. A systems approach was used to ensure that the key stakeholders including households, community and extension organizations were included. The study was initiated with a well-being analysis of community households to identify poorer households before exploring impacts of RBFSP on poorer producing households (RF) compared to non-producing (NRF) households based on one-off and longitudinal surveys. Livelihoods impacts on other actors linked directly and indirectly with RBFSP were also investigated. The adoption process of RBFSP at the household level and the cost-effectiveness of its promotion were assessed. Impact studies at the household level showed that RF households were significantly larger and had lower levels of formal education than NRF. Adoption of RBFSP had improved practical skills and hence substantially improved human capital in RF households. RF households tended to have more of their ricefish plots located adjacent to their households. Poor and intermediate adopters had smaller riceplots than better-off households but higher seed production efficiencies (poor-315.1 kg fingerlings/ha; intermediate-419.1 kg fingerlings/ha) than better-off households (294.6 kg fingerlings/ha). In addition to direct consumption of large fingerlings, RF households restocked them for further growth in their household ponds in doing so increasing yields by 60%. Fish consumption increased substantially in RF households based on their own production reducing their dependency on purchase from markets. The year round longitudinal survey revealed that activities for RBFSP were compatible with their existing rice-based agriculture activities for household members including men, women and children. The relatively limited income from fingerling production improved cash flow in the low income months. Consumption of large size fingerlings from ricefields provided nutrient dense food in the ‘hungry gap’ months when supplies of wild fish were poor, smoothing consumption. Apart from RF households, RBFSP extended its livelihoods impacts to a wide range of actors in and around the seed producing community. Poor fry traders were found to be key actors in the spread and support of RBFSP. On average fry traders supplied fingerlings to 35 foodfish producers within a mean distance of 5 Km from producing households in a community where RBFSP was well established. The end users (foodfish producers) included households with their own ponds, ponds with multiple ownership and larger waterbodies leased by small groups. Locally available RFBSP juveniles were attractive to each of these groups, supplementing hatchery derived seed. A large number of complex socio-cultural and technical factors were related to household level adoption of RBFSP. The major factors included use of cash generated to prevent distress sales of rice; lack of requirement to use pesticide in ricefields; meeting the household consumption demand; capacity to restock fingerlings in ponds; lack of any negative effects on rice production; increased non-stocked fish production in riceplots; simplicity of the technology; ease of fish harvest from riceplots; increased ability to gift fingerlings/foodfish to relatives and neighbours; more efficient use of both riceplot and irrigation pumps. The most important reasons for households not attempting or quickly rejecting RBFSP were labour conflicts with other activities. However, lost access to the riceplots through changes in tenure was the most common cause of late rejection by households who had practiced RBFSP for several years after withdrawal of CARE support. Location of fish seed producing plots close to the homesteads facilitated household women to contribute to seed production activities through feeding and looking after fish. Women were able to decide and control resources generated from fingerling sales as well as choosing to gift fingerlings to their relatives. Informal transfer of fingerlings in this way stimulated spread of RBFSP. Decentralised fish seed production was promoted through FFS very cost effectively. The introduction of an improved strain of Nile tilapia (GIFT) broodfish greatly enhanced the returns from decentralised seed production based on common carp alone. High levels of secondary adoption improved benefits from promoting RBFSP. The major benefit derived from the improved returns to food fish farmers using locally produced seed. Higher levels of net present value (NPV) and benefit cost ratio (BCR) were achieved based on promotion of mixed-sex tilapia in RBFSP than mono-sex tilapia produced in a large scale central hatchery. Cost-effectiveness in terms of multiplier development impacts on ramification of secondary adopters and, income of fry traders and foodfish producers, RBFSP also showed better performance than a mono-sex tilapia hatchery.