Graph Signal Processing: Overview, Challenges and Applications

Research in Graph Signal Processing (GSP) aims to develop tools for processing data defined on irregular graph domains. In this paper we first provide an overview of core ideas in GSP and their connection to conventional digital signal processing. We then summarize recent developments in developing basic GSP tools, including methods for sampling, filtering or graph learning. Next, we review progress in several application areas using GSP, including processing and analysis of sensor network data, biological data, and applications to image processing and machine learning. We finish by providing a brief historical perspective to highlight how concepts recently developed in GSP build on top of prior research in other areas.Comment: To appear, Proceedings of the IEE

A Survey of Graph Neural Networks for Social Recommender Systems

Social recommender systems (SocialRS) simultaneously leverage user-to-item interactions as well as user-to-user social relations for the task of generating item recommendations to users. Additionally exploiting social relations is clearly effective in understanding users' tastes due to the effects of homophily and social influence. For this reason, SocialRS has increasingly attracted attention. In particular, with the advance of Graph Neural Networks (GNN), many GNN-based SocialRS methods have been developed recently. Therefore, we conduct a comprehensive and systematic review of the literature on GNN-based SocialRS. In this survey, we first identify 80 papers on GNN-based SocialRS after annotating 2151 papers by following the PRISMA framework (Preferred Reporting Items for Systematic Reviews and Meta-Analysis). Then, we comprehensively review them in terms of their inputs and architectures to propose a novel taxonomy: (1) input taxonomy includes 5 groups of input type notations and 7 groups of input representation notations; (2) architecture taxonomy includes 8 groups of GNN encoder, 2 groups of decoder, and 12 groups of loss function notations. We classify the GNN-based SocialRS methods into several categories as per the taxonomy and describe their details. Furthermore, we summarize the benchmark datasets and metrics widely used to evaluate the GNN-based SocialRS methods. Finally, we conclude this survey by presenting some future research directions.Comment: GitHub repository with the curated list of papers: https://github.com/claws-lab/awesome-GNN-social-recsy

How Graph Convolutions Amplify Popularity Bias for Recommendation?

Graph convolutional networks (GCNs) have become prevalent in recommender system (RS) due to their superiority in modeling collaborative patterns. Although improving the overall accuracy, GCNs unfortunately amplify popularity bias -- tail items are less likely to be recommended. This effect prevents the GCN-based RS from making precise and fair recommendations, decreasing the effectiveness of recommender systems in the long run. In this paper, we investigate how graph convolutions amplify the popularity bias in RS. Through theoretical analyses, we identify two fundamental factors: (1) with graph convolution (\textit{i.e.,} neighborhood aggregation), popular items exert larger influence than tail items on neighbor users, making the users move towards popular items in the representation space; (2) after multiple times of graph convolution, popular items would affect more high-order neighbors and become more influential. The two points make popular items get closer to almost users and thus being recommended more frequently. To rectify this, we propose to estimate the amplified effect of popular nodes on each node's representation, and intervene the effect after each graph convolution. Specifically, we adopt clustering to discover highly-influential nodes and estimate the amplification effect of each node, then remove the effect from the node embeddings at each graph convolution layer. Our method is simple and generic -- it can be used in the inference stage to correct existing models rather than training a new model from scratch, and can be applied to various GCN models. We demonstrate our method on two representative GCN backbones LightGCN and UltraGCN, verifying its ability in improving the recommendations of tail items without sacrificing the performance of popular items. Codes are open-sourced \footnote{https://github.com/MEICRS/DAP}.Comment: Accepted by Frontiers of Computer Scienc

How much do we know about the User-Item Matrix?: Deep Feature Extraction for Recommendation

Collaborative filtering-based recommender systems typically operate on a high-dimensional sparse user-item matrix. Matrix completion is one of the most common formulations where rows and columns represent users and items, and predicting user’s ratings in items corresponds to filling in the missing entries of the matrix. In practice, it is a very challenging task to predict one's interest based on millions of other users having each seen a small subset of thousands of items. We considered how to extract the key features of users and items in the rating matrix to capture their features in a low-dimensional vector and how to create embeddings that well represent the characteristics of users and items by exploring what kind of user/item information to use in the matrix. However, recent studies have focused on utilising side information, such as user's age or movie's genre, but it is not always available and is hard to extract. More importantly, there has been no recent research on how to efficiently extract the important latent features from a sparse data matrix with no side information (1st problem). The next (2nd) problem is that most matrix completion techniques have mainly focused on semantic similarity between users and items with data structure transformation from a rating matrix to a user/item similarity matrix or a graph, neglecting the position of each element (user, item and rating) in the matrix. However, we think that a position is one of the fundamental points in matrix completion, since a specific point to be filled is presented based on the positions of its row and column in the matrix. In order to address the first (1st) problem, we aim to generalise and represent a high-dimensional sparse user-item matrix entry into a low-dimensional space with a small number of important features, and propose a Global-Local Kernel-based matrix completion framework, named GLocal-K, which is divided into two major stages. First, we pre-train an autoencoder with the local kernelised weight matrix, which transforms the data from one space into the feature space by using a 2d-RBF kernel. Then, the pre-trained autoencoder is fine-tuned with the rating matrix, produced by a convolution-based global kernel, which captures the characteristics of each item. GLocal-K outperforms the state-of-the-art baselines on three collaborative filtering benchmarks. However, it cannot show its superior feature extraction ability when the data is very large or too extremely sparse. For the aforementioned second (2nd) problem and the GLocal-K's limitation, we propose a novel position-enhanced user/item representation training model for recommendation, SUPER-Rec. We first capture the rating position in a matrix using relative positional rating encoding and store the position-enhanced rating information and its user-item relationship to a fixed dimension of embedding that is not affected by the matrix size. Then, we apply the trained position-enhanced user and item representations to the simplest traditional machine learning models to highlight the pure novelty of the SUPER-Rec representation. We contribute to the first formal introduction and quantitative analysis of the position-enhanced user/item representation in the recommendation domain and produce a principled discussion about SUPER-Rec with the incredibly excellent RMSE/MAE/NDCG/AUC results (i.e., both rating and ranking prediction accuracy) by an enormous margin compared with various state-of-the-art matrix completion models on both explicit and implicit feedback datasets. For example, SUPER-Rec showed the 28.2% RMSE error decrease in ML-1M compared to the best baseline, while the error decrease by 0.3% to 4.1% was prevalent among all the baselines

A survey of context-aware recommendation schemes in event-based social networks

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. In recent years, Event-based social network (EBSN) applications, such as Meetup and DoubanEvent, have received popularity and rapid growth. They provide convenient online platforms for users to create, publish, and organize social events, which will be held in physical places. Additionally, they not only support typical online social networking facilities (e.g., sharing comments and photos), but also promote face-to-face offline social interactions. To provide better service for users, Context-Aware Recommender Systems (CARS) in EBSNs have recently been singled out as a fascinating area of research. CARS in EBSNs provide the suitable recommendation to target users by incorporating the contextual factors into the recommendation process. This paper provides an overview on the development of CARS in EBSNs. We begin by illustrating the concept of the term context and the paradigms of conventional context-aware recommendation process. Subsequently, we introduce the formal definition of an EBSN, the characteristics of EBSNs, the challenges that are faced by CARS in EBSNs, and the implementation process of CARS in EBSNs. We also investigate which contextual factors are considered and how they are represented in the recommendation process. Next, we focus on the state-of-the-art computational techniques regarding CARS in EBSNs. We also overview the datasets and evaluation metrics for evaluation in this research area, and discuss the applications of context-aware recommendation in EBSNs. Finally, we point out research opportunities for the research community