To solve the lexical gap problem, most researchers focused on translation-based approaches since the relationships between words (or phrases) can be explicitly modeled through word-to-word (or phrases) translation probabilities (Jeon et al., 2005;Riezler et al., 2007;Xue et al., 2008;Lee et al., 2008;Bernhard and Gurevych, 2009;Singh, 2012).
while the term-topic matrix U p can be represented as Np ] ∈ R (Ks+Kp)×Np be the topic-question matrix corresponding to category c p , in which each column denotes the question representation in the topic space.