In our experiment, some widely used text clustering methods are compared with our approach. Besides K-means, Skip-thought Vectors, Recursive Neural Network and Paragraph Vector based clustering methods, four baseline clustering methods are directly based on the popular unsupervised dimensionality reduction methods as described in Section SECREF11 .  
 Question: Which popular clustering methods did they experiment with?
K-means, Skip-thought Vectors, Recursive Neural Network and Paragraph Vector based clustering methods