Computing the topic distribution over a document and assigning it to the sentences serves to tie the sentences together in the document context.
to obtain the lexical probability conditioned on topic distribution, we first compute the expected count e zn (e, f ) of a word pair under topic z n : where c j (•) denotes the number of occurrences of the word pair in sentence x j , and then compute: e e zn (e, f ) we will introduce 2•K new word translation tables, one for each p zn (e|f ) and p zn (f |e), and as many new corresponding features f zn (e|f ), f zn (f |e).