We follow a long tradition of older (Finch and Chater, 1992;Schütze, 1993;Schütze, 1995) and newer (Huang and Yates, 2009) work on creating distributional features for POS tagging based on local left and right neighbors.
the i th entry x i of f left (w) is the weighted number of times that the indicator word c i occurs immediately to the left of w: where c i is the word with frequency rank i in the corpus, freq (bigram(c i , w)) is the number of times the bigram "c i w" occurs in the corpus and we weight the non-zero frequencies logarithmically: tf(x) = 1 + log(x).