N ov 2 00 6 Some statistics on permutations avoiding generalized patterns ∗

In the last decade a huge amount of articles has been published studying pattern avoidance on permutations. From the point of view of enumeration, typically one tries to count permutations avoiding certain patterns according to their lengths. Here we tackle the problem of refining this enumeration by considering the statistics “first/last entry”. We give complete results for every generalized patterns of type (1, 2) or (2, 1) as well as for some cases of permutations avoiding a pair of generalized patterns of the above types.