Two descent statistics over 321-avoiding centrosymmetric involutions

Centrosymmetric involutions in the symmetric group S_{2n} are permutations pi such that pi = pi^{- 1} and pi(i)+ pi(2n + 1 - i) = 2n + 1 for all i, and they are in bijection with involutions of the hyperoctahedral group. We describe the distribu- tion of some natural descent statistics on 321-avoiding centrosymmetric involutions, including the number of descents in the rst half of the involution, and the sum of the positions of these descents. Our results are based on two new bijections, one between centrosymmetric involutions in S_{2n} and subsets of {1; ...; n}, and another one showing that certain statistics on Young diagrams that fit inside a rectangle are equidistributed. We also use the latter bijection to rene a known result stating that the distribution of the major index on 321-avoiding involutions is given by the q-analogue of the central binomial coefficients.