The optimal Λ can be obtained by quasi-Newton methods using the above L Λ and ∂L Λ ∂λ i,j , and we use L-BFGS (Liu and Nocedal, 1989) for this purpose 2 .
the calculation is intractable because Z Λ (w l ) (see Equation 9)in Equation (16) and a term in Equation 17contain summations over all the possible POS tags.