Sequential Recommendation predicts the next item based on users' past behaviors, but sparse interaction data makes user preferences hard to learn. Recently, contrastive learning has shown promise in this area. It augments data to form positive pairs and maximizing their similarity, allowing the model to learn more generalizable user interests. However, they mainly adopt uniform augmentation and alignment to all sequences, ignoring the challenges arising from their distinct interest structure, namely semantic discrepancy and semantic bias. In this paper, we first study the impact of augmentation on sequence's semantic through Interest Entropy, which measures the diversity and density of interest distribution. Our finding shows only a small fraction of sequences are stable under perturbation. These sequences mainly exhibit low or high entropy, reflecting focused or casual interests. This limits the effectiveness of contrastive learning, which relies on semantically consistent positive pairs. Furthermore, with spectral analysis, we show that positive alignment may cause low-entropy sequences to overlook niche interests, while high-entropy sequences may amplify interest-irrelevant signals, which we term semantic bias. Finally, based on Interest Entropy, we propose IERec, a simple yet effective mutual retrieval augmented contrastive learning method that mitigates the above issues in a unified manner. For each anchor sequence (those with low or high entropy), we retrieve semantically similar sequences with complementary entropy, and concatenate them to form a positive view. Sequences that are easily affected, mainly those with medium entropy, are excluded from augmentation. This approach can avoid harmful semantic discrepancy of positive pairs and reduce the effect of the semantic bias, leading to improved performance. Moreover, using interest entropy to guide contrastive learning can further improve existing CL-based SR methods.

Sequential Recommendation utilizes interaction history to uncover users' dynamic interest changes and recommend the most relevant items for their next interaction. In recent years, multi-behavior modeling and multi-interest modeling have been hot research topics. Although multi-behavior and multi-interest methods have strengths in their respective domains, both have limitations. Multi-behavior methods focus excessively on target behavior recommendation (i.e., purchase) without sufficiently leveraging auxiliary behavior interactions (i.e., click) to discern users' multi-faced interests, leading to suboptimal recommendation quality. Meanwhile, existing multi-interest methods overlook the distinct user interests behind multi-behavior when extracting interests, resulting in inaccurate interest modeling. Combining the two can not only facilitate sophisticated modeling of complex user interests but also deepen understanding of multi-behavior interactions, achieving synergistic effects. In this paper, we propose a novel approach called Multi-Interest Self-Supervised Learning (MISSL) that precisely unifies multi-behavior and multi-interest modeling to obtain more comprehensive and accurate user profiles. MISSL utilizes a hypergraph transformer network to extract behavior-specific and shared interests followed by multi-interest self-supervised learning to refine item and interest representations. Additionally, a behavior-aware training task is incorporated to enhance model stability during training. Extensive experiments on benchmark datasets demonstrate that MISSL outperforms baseline methods. The source code for MISSL is available at: https://github.com/qianlima-Iab/MISSL.

Irregularly Sampled Medical Time Series (ISMTS) are commonly found in the healthcare domain, where different variables exhibit unique temporal patterns while interrelated. However, many existing methods fail to efficiently consider the differences and correlations among medical variables together, leading to inadequate capture of fine-grained features at the variable level in ISMTS. We propose Knowledge-Empowered Dynamic Graph Network (KEDGN), a graph neural network empowered by variables' textual medical knowledge, aiming to model variable-specific temporal dependencies and inter-variable dependencies in ISMTS. Specifically, we leverage a pre-trained language model to extract semantic representations for each variable from their textual descriptions of medical properties, forming an overall semantic view among variables from a medical perspective. Based on this, we allocate variable-specific parameter spaces to capture variable-specific temporal patterns and generate a complete variable graph to measure medical correlations among variables. Additionally, we employ a density-aware mechanism to dynamically adjust the variable graph at different timestamps, adapting to the time-varying correlations among variables in ISMTS. The variable-specific parameter spaces and dynamic graphs are injected into the graph convolutional recurrent network to capture intra-variable and inter-variable dependencies in ISMTS together. Experiment results on four healthcare datasets demonstrate that KEDGN significantly outperforms existing methods.