Data clustering and representation learning play an indispensable role in data science. They are very useful to explore massive data in many fields, including information retrieval, natural language processing, bioinformatics, recommender systems, and computer vision. Despite their success, most existing clustering methods are severely challenged by the data generated by modern applications, which are typically high dimensional, noisy, heterogeneous, and sparse or even collected from multiple sources or represented by multiple views where each describes a perspective of the data. This has driven many researchers to investigate new effective clustering models to overcome these difficulties. One promising category of such models relies on representation learning. Indeed, learning a good data representation is crucial for clustering algorithms, and combining the two tasks is a common way of exploring this type of data. The idea is to embed the original data into a low dimensional latent space and then perform clustering on this new space. However, both tasks can be carried out sequentially or jointly. Many clustering algorithms, including deep learning versions, are based on these two modes of combining the two tasks.

Online marketplaces are able to offer a staggering array of products that no physical store can match. While this makes it more likely for customers to find what they want, in order for online providers to ensure a smooth and efficient user experience, they must maintain well-organized catalogs, which depends greatly on the availability of per-product attribute values such as color, material, brand, to name a few. Unfortunately, such information is often incomplete or even missing in practice, and therefore we have to resort to predictive models as well as other sources of information to impute missing attribute values.

Data sparsity is a long-standing challenge in recommender systems. Among existing approaches to alleviate this problem, cross-domain recommendation consists in leveraging knowledge from a source domain or category (e.g., Movies) to improve item recommendation in a target domain (e.g., Books). In this work, we advocate a probabilistic approach to cross-domain recommendation and rely on variational autoencoders (VAEs) as our latent variable models. More precisely, we assume that we have access to a VAE trained on the source domain that we seek to leverage to improve preference modeling in the target domain. To this end, we propose a model which learns to fit the target observations and align its hidden space with the source latent space jointly. Since we model the latent spaces by the variational posteriors, we operate at this level, and in particular, we investigate two approaches, namely rigid and soft alignments. In the former scenario, the variational model in the target domain is set equal to the source variational model. That is, we only learn a generative model in the target domain. In the soft-alignment scenario, the target VAE has its variational model, but which is encouraged to look like its source counterpart. We analyze the proposed objectives theoretically and conduct extensive experiments to illustrate the benefit of our contribution. Empirical results on six real-world datasets show that the proposed models outperform several comparable cross-domain recommendation models.

Recommender systems typically learn from user-item preference data such as ratings and clicks. This information is sparse in nature, i.e., observed user-item preferences often represent less than 5% of possible interactions. One promising direction to alleviate data sparsity is to leverage auxiliary information that may encode additional clues on how users consume items. Examples of such data (referred to as modalities) are social networks, item’s descriptive text, product images. The objective of this tutorial is to offer a comprehensive review of recent advances to represent, transform and incorporate the different modalities into recommendation models. Moreover, through practical hands-on sessions, we consider cross model/modality comparisons to investigate the importance of different methods and modalities. The hands-on exercises are conducted with Cornac (https://cornac.preferred.ai ), a comparative framework for multimodal recommender systems. The materials are made available on https://preferred.ai/recsys21-tutorial/.

Preference data is a form of dyadic data, with measurements associated with pairs of elements arising from two discrete sets of objects. These are users and items, as well as their interactions, e.g., ratings. We are interested in learning representations for both sets of objects, i.e., users and items, to predict unknown pairwise interactions. Motivated by the recent successes of deep latent variable models, we propose Bilateral Variational Autoencoder (BiVAE), which arises from a combination of a generative model of dyadic data with two inference models, user- and item-based, parameterized by neural networks. Interestingly, our model can take the form of a Bayesian variational autoencoder either on the user or item side. As opposed to the vanilla VAE model, BiVAE is "bilateral'', in that users and items are treated similarly, making it more apt for two-way or dyadic data. While theoretically sound, we formally show that, similarly to VAE, our model might suffer from an over-regularized latent space. This issue, known as posterior collapse in the VAE literature, may appear due to assuming an over-simplified prior (isotropic Gaussian) over the latent space. Hence, we further propose a mitigation of this issue by introducing constrained adaptive prior (CAP) for learning user- and item-dependent prior distributions. Empirical results on several real-world datasets show that the proposed model outperforms conventional VAE and other comparative collaborative filtering models in terms of item recommendation. Moreover, the proposed CAP further boosts the performance of BiVAE. An implementation of BiVAE is available on Cornac recommender library.

Semi-Non Negative Matrix Factorization (Semi-NMF) is one of the most popular extensions of NMF, it extends the applicable range of NMF models, to data having mixed signs, as well as strengthens their relation to clustering. However, Semi-NMF has been found to perform somewhat less than NMF, in terms of clustering, when applied to positive data such as text, which we focus on. Inspired by the recent success of neural word embedding models, e.g., word2vec, in learning high quality real valued vector representations of words, we propose to integrate a word embedding model into Semi-NMF. This allows Semi-NMF to capture more semantic relationships among words and, thereby, to infer document factors that are even better for clustering. The combination of Semi-NMF and word embedding noticeably improves the performance of NMF models, in terms of both clustering and embedding, as illustrated in our experiments.

Co-clustering has proven effective to deal with high dimensional sparse data, such as document-term matrices encountered in text mining. Apart from being high dimensional and sparse, the data sets from the aforementioned domain are also directional in nature. Most existing co-clustering approaches are, however, based on popular modelling assumptions, such as Gaussian or Multinomial, which are inadequate for directional data. Moreover, it is well known that, due to high dimensionality and sparsity, co-clustering approaches, like one-sided clustering methods, tend to generate highly skewed solutions with very unbalanced or even empty clusters, especially when the number of required clusters is large. In this paper, we rely on the recently proposed block von Mises-Fisher mixture model (dbmovMFs), which constitutes a general framework for co-clustering directional data distributed on the surface of a unit hypersphere, i.e, L2 normalized data. In order to overcome the above difficulties, we propose to modify dbmovMFs in a principled way by introducing a conscience mechanism which discourages bad local solutions having empty or very small/large clusters. This gives rise to a new scalable co-clustering algorithm which is guaranteed to increase monotonically a spherical k-means like criterion by intertwining row and column clusterings at each step. Moreover, empirical results, on several real-world datasets, provide strong support for the effectiveness of the proposed approach.

Document clustering is central in modern information retrieval applications. Among existing models, non-negative-matrix factorization (NMF) approaches have proven effective for this task. However, NMF approaches, like other models in this context, exhibit a major drawback, namely they use the bag-of-word representation and, thus, do not account for the sequential order in which words occur in documents. This is an important issue since it may result in a significant loss of semantics. In this paper, we aim to address the above issue and propose a new model which successfully integrates a word embedding model, word2vec, into an NMF framework so as to leverage the semantic relationships between words. Empirical results, on several real-world datasets, demonstrate the benefits of our model in terms of text document clustering as well as document/word embedding.

Co-clustering is more useful than one-sided clustering when dealing with high dimensional sparse data. We propose to address the aim of document clustering with a generative model-based co-clustering approach. To this end, we rely on a particular mixture of von Mises-Fisher distributions and propose a new parsimonious model allowing to reveal a block diagonal structure as well as a good partitioning of documents and terms. Then, by setting the estimate of the model parameters under the maximum likelihood (ML) approach, we derive three novel co-clustering algorithms: a soft one and two stochastic variants. Empirical results on numerous simulated and real-world datasets, demonstrate the advantages of our approach to model and co-cluster high dimensional sparse data.