With the rapid advancement of mobile networks and the widespread use of mobile devices, spatial crowdsourcing, which involves assigning location-based tasks to mobile workers, has gained significant attention. However, most existing research focuses on task assignment at the current moment, overlooking the fluctuating demand and supply between tasks and workers over time. To address this issue, we introduce an adaptive task assignment problem, which aims to maximize the number of assigned tasks by dynamically adjusting task assignments in response to changing demand and supply. We develop a spatial crowdsourcing framework, namely demand-based adaptive task assignment with dynamic worker availability windows, which consists of two components including task demand prediction and task assignment. In the first component, we construct a graph adjacency matrix representing the demand dependency relationships in different regions and employ a multivariate time series learning approach to predict future task demands. In the task assignment component, we adjust tasks to workers based on these predictions, worker availability windows, and the current task assignments, where each worker has an availability window that indicates the time periods they are available for task assignments. To reduce the search space of task assignments and be efficient, we propose a worker dependency separation approach based on graph partition and a task value function with reinforcement learning. Experiments on real data demonstrate that our proposals are both effective and efficient.

With the rapid evolution of sensing techniques and the proliferation of mobile devices, spatial crowdsourcing (SC) has gained significant attention in both academia and industry. SC involves assigning location-based tasks to mobile workers, with task recommendation playing a key role in helping workers identify suitable and appealing tasks. However, most existing studies focus on task completion rate, worker satisfaction, or efficiency, without consideration of the environmental impact, e.g., pollutant emissions from the increased vehicle usage associated with SC applications like Uber, Lyft, and FoodPanda. In this study, we consider a novel problem of sustainable task recommendation in SC, which aims to minimize the environmental footprint (i.e., pollution) while maintaining acceptable levels of task completion, worker satisfaction, and overall task recommendation efficiency. We develop an innovative Sustainability-Oriented Task Recommendation framework encompassing two major components: speed-driven pollutant emission estimation and task recommendation. Specifically, the pollutant emission estimation component aims to estimate future pollutant emissions based on worker trajectories and speeds, using a context-enhanced spatio-temporal network for road speed prediction. In the task recommendation component, we provide a completion-sensitive recommendation algorithm to maximize the expected number of completed tasks. Further, we design an efficient emission-optimized KM ranking algorithm to minimize emissions. Experiments on real data offer insight into the effectiveness and efficiency of the proposals, providing valuable insights into its potential for sustainable spatial crowdsourcing.

Counterfactual learning is emerging as an important paradigm, rooted in causality, which promises to alleviate common issues of graph neural networks (GNNs), such as fairness and interpretability. However, as in many real-world application domains where conducting randomized controlled trials is impractical, one has to rely on available observational (factual) data to detect counterfactuals. In this paper, we introduce and tackle the problem of searching for counterfactual evidences for the GNN-based node classification task. A counterfactual evidence is a pair of nodes such that, regardless they exhibit great similarity both in the features and in their neighborhood subgraph structures, they are classified differently by the GNN. We develop effective and efficient search algorithms and a novel indexing solution that leverages both node features and structural information to identify counterfactual evidences, and generalizes beyond any specific GNN. Through various downstream applications, we demonstrate the potential of counterfactual evidences to enhance fairness and accuracy of GNNs.

This paper introduces a new class of explanation structures, called robust counterfactual witnesses (RCWs), to provide robust, both counterfactual and factual explanations for graph neural networks. Given a graph neural network$\mathcal{M}$, a robust counterfactual witness refers to the fraction of a graph$G$that are counterfactual and factual explanation of the results of$\mathcal{M}$over$G$, but also remains so for any “disturbed”$G$by flipping up to$k$of its node pairs. We establish the hardness results, from tractable results to co-NP-hardness, for verifying and generating robust counterfactual witnesses. We study such structures for GNN-based node classification, and present efficient algorithms to verify and generate RCWs. We also provide a parallel algorithm to verify and generate RCWs for large graphs with scalability guarantees. We experimentally verify our explanation generation process for benchmark datasets, and showcase their applications.

Generating explanations for graph neural networks (GNNs) has been studied to understand their behaviors in analytical tasks such as graph classification. Existing approaches aim to understand the overall results of GNNs rather than providing explanations for specific class labels of interest, and may return explanation structures that are hard to access, nor directly queryable. We propose GVEX, a novel paradigm that generates Graph Views for GNN EXplanation. (1) We design a two-tier explanation structure called explanation views. An explanation view consists of a set of graph patterns and a set of induced explanation subgraphs. Given a database G of multiple graphs and a specific class label l assigned by a GNN-based classifier M, it concisely describes the fraction of G that best explains why l is assigned by M. (2) We propose quality measures and formulate an optimization problem to compute optimal explanation views for GNN explanation. We show that the problem is Σ2P-hard. (3) We present two algorithms. The first one follows an explain-and-summarize strategy that first generates high-quality explanation subgraphs which best explain GNNs in terms of feature influence maximization, and then performs a summarization step to generate patterns. We show that this strategy provides an approximation ratio of 1/2. Our second algorithm performs a single-pass to an input node stream in batches to incrementally maintain explanation views, having an anytime quality guarantee of 1/4-approximation. Using real-world benchmark data, we experimentally demonstrate the effectiveness, efficiency, and scalability of GVEX. Through case studies, we showcase the practical applications of GVEX.

With the proliferation of wireless and mobile devices, Spatial Crowdsourcing (SC) attracts increasing attention, where task assignment plays a critically important role. However, recent task assignment solutions in SC often assume that data is stored in a central station while ignoring the issue of privacy leakage. To enable decentralized training and privacy protection, we propose a federated task assignment framework with personalized location-preference learning, which performs efficient task assignment while keeping the data decentralized and private in each platform center (e.g., a delivery center of an SC company). The framework consists of two phases: personalized federated location-preference learning and task assignment. Specifically, in the first phase, we design a personalized location-preference learning model for each platform center by simultaneously considering the location information and data heterogeneity across platform centers. Based on workers' location preference, the task assignment phase aims to achieve effective and efficient task assignment by means of the Kuhn-Munkres (KM) algorithm and the newly proposed conditional degree-reduction algorithm. Extensive experiments on real-world data show the effectiveness of the proposed framework.

With the rapid development of mobile networks and the widespread usage of mobile devices, Location-Based Adver-tising (LBA), which allows an advertiser to promote products or services to targeted customers in a suitable location, has drawn increasing attention. Recommending an optimal location by delivering appealing advertisements to potential customers is crucial for the advertiser. Existing recommendation models (such as collaborative filtering) are insufficient for solving the data sparsity and cold-start issue (e.g., no historical advertisement records in new domains) in LBA problems. To tackle the defi-ciency mentioned above, we propose a novel location-based ad-vertising recommendation framework: CityCross. The CityCross framework consists of a data extraction module and a learning module. The data extraction module conducts commercial and POI feature extractions from the LBA platform, and Gaode Map, respectively. The learning module is dedicated to learning the relevant knowledge of advertisement in a new domain by utilizing the attention-based semantic information, cross-city knowledge association, and the local neighbors' knowledge. The top-k locations are identified by a modified linear regression model based on the learned knowledge. Finally, we conduct extensive experiments on two real datasets to verify the superiority of the proposed approach.