I. Introduction
Collecting labeled examples can be expensive and difficult in many data analysis and mining applications, such as medical diagnosis and analysis, human action recognition, image retrieval, and remote sensing image classification [1]–[5]. However, there is an abundance of available unlabeled data. To alleviate this issue, researchers have tried to use plentiful unlabeled data together with limited labeled data, and thus developed semisupervised learning (SSL) methods [6]. It has been confirmed that SSL not only avoids wasting data sources but also improves upon the generalization capability of traditional supervised learning methods.