Number of permutations in S_n with a certain property.

Terms

a(0) =1a(1) =0a(2) =1a(3) =3a(4) =5a(5) =12a(6) =22a(7) =50a(8) =95a(9) =210a(10) =406a(11) =882a(12) =1722a(13) =3696a(14) =7260a(15) =15444a(16) =30459a(17) =64350a(18) =127270a(19) =267410a(20) =529958a(21) =1108536a(22) =2200276a(23) =4585308a(24) =9111830a(25) =18929092a(26) =37650172a(27) =78004500a(28) =155266100a(29) =320932800