1 Introduction
Exploratory search is an increasingly important activity yet challenging for Web searchers. In exploratory search, the searcher is unfamiliar with their problem domain, unsures about the ways to achieve their goal, or lacks a well-defined goal [1]. To support exploratory search, the search system is required not only to provide accurate search results, but also to help searchers explore related and novel aspects. Therefore exploratory search system needs an effective query recommendation method to resolve this problem.