To solve this problem, we introduce the topic mapping probability P (t f out |t f in ) to map the in-domain phrase-topic distribution into the one in the out-domain topic space.
to be specific, we obtain the out-of-domain phrase-topic distribution P (t f out |f ) as follows: formula (4) can be further refined as the following formula: Next we will give detailed descriptions of the calculation methods for the three probability distributions mentioned in formula (6).