Deep Learning for Recommender Systems

The widespread adoption of the Internet has led to an explosion in the number of choices available to consumers. Users begin to expect personalized content in modern E-commerce, entertainment and social media platforms. Recommender Systems (RS) provide a critical solution to this problem by maintaining user engagement and satisfaction with personalized content. Traditional RS techniques are often linear limiting the expressivity required to model complex user-item interactions and require extensive handcrafted features from domain experts. Deep learning demonstrated significant breakthroughs in solving problems that have alluded the artificial intelligence community for many years advancing state-of-the-art results in domains such as computer vision and natural language processing. The recommender domain consists of heterogeneous and semantically rich data such as unstructured text (e.g. product descriptions), categorical attributes (e.g. genre of a movie), and user-item feedback (e.g. purchases). Deep learning can automatically capture the intricate structure of user preferences by encoding learned feature representations from high dimensional data. In this thesis, we explore five novel applications of deep learning-based techniques to address top-n recommendation. First, we propose Collaborative Memory Network, which unifies the strengths of the latent factor model and neighborhood-based methods inspired by Memory Networks to address collaborative filtering with implicit feedback. Second, we propose Neural Semantic Personalized Ranking, a novel probabilistic generative modeling approach to integrate deep neural network with pairwise ranking for the item cold-start problem. Third, we propose Attentive Contextual Denoising Autoencoder augmented with a context-driven attention mechanism to integrate arbitrary user and item attributes. Fourth, we propose a flexible encoder-decoder architecture called Neural Citation Network, embodying a powerful max time delay neural network encoder augmented with an attention mechanism and author networks to address context-aware citation recommendation. Finally, we propose a generic framework to perform conversational movie recommendations which leverages transfer learning to infer user preferences from natural language. Comprehensive experiments validate the effectiveness of all five proposed models against competitive baseline methods and demonstrate the successful adaptation of deep learning-based techniques to the recommendation domain

Exploring attributes, sequences, and time in Recommender Systems: From classical to Point-of-Interest recommendation

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Escuela Politécnica Superior, Departamento de Ingenieria Informática. Fecha de lectura: 08-07-2021Since the emergence of the Internet and the spread of digital communications throughout the world, the amount of data stored on the Web has been growing exponentially. In this new digital era, a large number of companies have emerged with the purpose of ltering the information available on the web and provide users with interesting items. The algorithms and models used to recommend these items are called Recommender Systems. These systems are applied to a large number of domains, from music, books, or movies to dating or Point-of-Interest (POI), which is an increasingly popular domain where users receive recommendations of di erent places when they arrive to a city. In this thesis, we focus on exploiting the use of contextual information, especially temporal and sequential data, and apply it in novel ways in both traditional and Point-of-Interest recommendation. We believe that this type of information can be used not only for creating new recommendation models but also for developing new metrics for analyzing the quality of these recommendations. In one of our rst contributions we propose di erent metrics, some of them derived from previously existing frameworks, using this contextual information. Besides, we also propose an intuitive algorithm that is able to provide recommendations to a target user by exploiting the last common interactions with other similar users of the system. At the same time, we conduct a comprehensive review of the algorithms that have been proposed in the area of POI recommendation between 2011 and 2019, identifying the common characteristics and methodologies used. Once this classi cation of the algorithms proposed to date is completed, we design a mechanism to recommend complete routes (not only independent POIs) to users, making use of reranking techniques. In addition, due to the great di culty of making recommendations in the POI domain, we propose the use of data aggregation techniques to use information from di erent cities to generate POI recommendations in a given target city. In the experimental work we present our approaches on di erent datasets belonging to both classical and POI recommendation. The results obtained in these experiments con rm the usefulness of our recommendation proposals, in terms of ranking accuracy and other dimensions like novelty, diversity, and coverage, and the appropriateness of our metrics for analyzing temporal information and biases in the recommendations producedDesde la aparici on de Internet y la difusi on de las redes de comunicaciones en todo el mundo, la cantidad de datos almacenados en la red ha crecido exponencialmente. En esta nueva era digital, han surgido un gran n umero de empresas con el objetivo de ltrar la informaci on disponible en la red y ofrecer a los usuarios art culos interesantes. Los algoritmos y modelos utilizados para recomendar estos art culos reciben el nombre de Sistemas de Recomendaci on. Estos sistemas se aplican a un gran n umero de dominios, desde m usica, libros o pel culas hasta las citas o los Puntos de Inter es (POIs, en ingl es), un dominio cada vez m as popular en el que los usuarios reciben recomendaciones de diferentes lugares cuando llegan a una ciudad. En esta tesis, nos centramos en explotar el uso de la informaci on contextual, especialmente los datos temporales y secuenciales, y aplicarla de forma novedosa tanto en la recomendaci on cl asica como en la recomendaci on de POIs. Creemos que este tipo de informaci on puede utilizarse no s olo para crear nuevos modelos de recomendaci on, sino tambi en para desarrollar nuevas m etricas para analizar la calidad de estas recomendaciones. En una de nuestras primeras contribuciones proponemos diferentes m etricas, algunas derivadas de formulaciones previamente existentes, utilizando esta informaci on contextual. Adem as, proponemos un algoritmo intuitivo que es capaz de proporcionar recomendaciones a un usuario objetivo explotando las ultimas interacciones comunes con otros usuarios similares del sistema. Al mismo tiempo, realizamos una revisi on exhaustiva de los algoritmos que se han propuesto en el a mbito de la recomendaci o n de POIs entre 2011 y 2019, identi cando las caracter sticas comunes y las metodolog as utilizadas. Una vez realizada esta clasi caci on de los algoritmos propuestos hasta la fecha, dise~namos un mecanismo para recomendar rutas completas (no s olo POIs independientes) a los usuarios, haciendo uso de t ecnicas de reranking. Adem as, debido a la gran di cultad de realizar recomendaciones en el ambito de los POIs, proponemos el uso de t ecnicas de agregaci on de datos para utilizar la informaci on de diferentes ciudades y generar recomendaciones de POIs en una determinada ciudad objetivo. En el trabajo experimental presentamos nuestros m etodos en diferentes conjuntos de datos tanto de recomendaci on cl asica como de POIs. Los resultados obtenidos en estos experimentos con rman la utilidad de nuestras propuestas de recomendaci on en t erminos de precisi on de ranking y de otras dimensiones como la novedad, la diversidad y la cobertura, y c omo de apropiadas son nuestras m etricas para analizar la informaci on temporal y los sesgos en las recomendaciones producida

Exploiting subsequence matching in Recommender Systems

Desde su surgimiento al inicio de la década de los 90, los sistemas de recomendación han experimentado un crecimiento exponencial empleándose en cada vez más aplicaciones debido a la utilidad que tienen para ayudar a los usuarios a elegir artículos en función de sus gustos y necesidades. Actualmente son indispensables en un gran número de empresas que ofrecen su servicio a través de Internet, el medio de intercambio de información más importante que existe. Por esta razón, la continua innovación en estos sistemas resulta imprescindible para poder efectuar recomendaciones que sean capaces de seguir sorprendiendo a los usuarios y mejorar las ya existentes. En este Trabajo Fin de Máster hemos realizado un estudio e investigación acerca del estado actual de estos sistemas, prestando especial atención a los sistemas de filtrado colaborativo basados en vecinos y los basados en contenido. No obstante, debido a las desventajas que puede tener cada sistema por separado normalmente en aplicaciones reales se emplean combinaciones de varios sistemas, creando recomendadores híbridos. Como apoyo a este estudio, se propone como aspecto novedoso el uso del algoritmo de la subcadena común más larga (LCS) para ser utilizada como medida de similitud entre usuarios, introduciendo además, una técnica general y transparente para generar secuencias haciendo uso tanto de información de contenido como de información colaborativa, pudiendo generar recomendadores híbridos de manera sencilla. Complementando a estos nuevos recomendadores, también detallamos otros parámetros auxiliares (confianza, preferencia, normalizaciones y distintas ordenaciones) que tienen como fin mejorar el rendimiento de estos sistemas basados en LCS. Por otro lado, además de la definición de estos nuevos recomendadores, el trabajo se complementa con resultados experimentales haciendo uso de tres conjuntos de datos conocidos en el área: Movielens, Lastfm y MovieTweetings. Cada uno de ellos estará orientado a explotar un aspecto específico de la generación de secuencias. Los resultados han sido obtenidos haciendo uso de métricas de ranking (Precisión, Recall, MAP o nDCG) y de novedad y diversidad (_-nDCG, EPC, EPD, Aggregate diversity, EILD y Gini). Los resultados han tenido como fin comparar el rendimiento de los recomendadores basados en la subsecuencia común más larga frente a otros recomendadores conocidos en el área. Como resumen, se ha observado que los recomendadores propuestos resultan altamente competitivos en las pruebas realizadas siendo incluso mejores en algunas ocasiones a otros recomendadores conocidos en el área, tanto en términos de métricas de ranking como de novedad y diversidad. No obstante, también se ha observado que, en algunos casos, el uso de recomendadores híbridos basados en la subsecuencia común más larga obtiene unos resultados peores que otras versiones puramente colaborativas. En cualquier caso, consideramos que es una propuesta con potencial para seguir siendo investigada.Since their inception in the early 1990s, recommender systems have experienced exponential growth as they are being used in a large number of applications because of their usefulness in helping users choose items based on their tastes and needs. Nowadays, they are indispensable in many companies that o er their service through the Internet, the most important method for information exchange. For this reason, continuous innovation in these systems is essential to make recommendations that are able to continue surprising users, while improving the existing ones. In this Master's Thesis, we have studied and researched on the current state of these systems, paying special attention to collaborative ltering based on neighborhood and content-based algorithms. However, due to the disadvantages that each system may have separately, combinations of these systems are often used in real applications, creating hybrid recommenders. To support this study, we propose the use of the longest common subsequence (LCS) algorithm as a novel aspect to be used as a similarity metric between users, also introducing a general and transparent technique to generate sequences using both content and collaborative information, allowing us to generate hybrid recommenders in a simple way. Complementing these new recommendations, we also detail other auxiliary parameters (con dence, preference, normalization functions, and di erent orderings), whose main goal is to improve the performance of these LCS-based systems. On the other hand, in addition to the de nition of these new recommenders, the study is complemented with experimental results using three well-known datasets in the area: Movielens, Lastfm and MovieTweetings. Each one of them will be oriented to exploit a speci c aspect of the sequence generation process. The results have been obtained by using ranking metrics (Precision, Recall, MAP, or nDCG) and novelty and diversity metrics ( -nDCG, EPC, EPD, Aggregate diversity, EILD, and Gini). With these experiments, we aimed at comparing the performance of recommenders based on the longest common subsequence against other well-known recommenders in the area. As a summary, we have observed in the experiments performed that the proposed recommenders are highly competitive, and sometimes they are even better than other recommenders known in the area, both in terms of ranking quality metrics, and novelty and diversity dimensions. However, we have also observed that, in some cases, the use of hybrid recommenders based on the longest common subsequence results in worse performance than other purely collaborative versions. In any case, we believe this is a proposal with enough potential to be worthy of further investigation

Learning by Fusing Heterogeneous Data

It has become increasingly common in science and technology to gather data about systems at different levels of granularity or from different perspectives. This often gives rise to data that are represented in totally different input spaces. A basic premise behind the study of learning from heterogeneous data is that in many such cases, there exists some correspondence among certain input dimensions of different input spaces. In our work we found that a key bottleneck that prevents us from better understanding and truly fusing heterogeneous data at large scales is identifying the kind of knowledge that can be transferred between related data views, entities and tasks. We develop interesting and accurate data fusion methods for predictive modeling, which reduce or entirely eliminate some of the basic feature engineering steps that were needed in the past when inferring prediction models from disparate data. In addition, our work has a wide range of applications of which we focus on those from molecular and systems biology: it can help us predict gene functions, forecast pharmacological actions of small chemicals, prioritize genes for further studies, mine disease associations, detect drug toxicity and regress cancer patient survival data. Another important aspect of our research is the study of latent factor models. We aim to design latent models with factorized parameters that simultaneously tackle multiple types of data heterogeneity, where data diversity spans across heterogeneous input spaces, multiple types of features, and a variety of related prediction tasks. Our algorithms are capable of retaining the relational structure of a data system during model inference, which turns out to be vital for good performance of data fusion in certain applications. Our recent work included the study of network inference from many potentially nonidentical data distributions and its application to cancer genomic data. We also model the epistasis, an important concept from genetics, and propose algorithms to efficiently find the ordering of genes in cellular pathways. A central topic of our Thesis is also the analysis of large data compendia as predictions about certain phenomena, such as associations between diseases and involvement of genes in a certain phenotype, are only possible when dealing with lots of data. Among others, we analyze 30 heterogeneous data sets to assess drug toxicity and over 40 human gene association data collections, the largest number of data sets considered by a collective latent factor model up to date. We also make interesting observations about deciding which data should be considered for fusion and develop a generic approach that can estimate the sensitivities between different data sets

Biometric Systems

Because of the accelerating progress in biometrics research and the latest nation-state threats to security, this book's publication is not only timely but also much needed. This volume contains seventeen peer-reviewed chapters reporting the state of the art in biometrics research: security issues, signature verification, fingerprint identification, wrist vascular biometrics, ear detection, face detection and identification (including a new survey of face recognition), person re-identification, electrocardiogram (ECT) recognition, and several multi-modal systems. This book will be a valuable resource for graduate students, engineers, and researchers interested in understanding and investigating this important field of study

Q(sqrt(-3))-Integral Points on a Mordell Curve

We use an extension of quadratic Chabauty to number fields,recently developed by the author with Balakrishnan, Besser and M ̈uller,combined with a sieving technique, to determine the integral points overQ(√−3) on the Mordell curve y2 = x3 − 4