Number of permutations of length n with at most two descents.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =23a(5) =93a(6) =360a(7) =1312a(8) =4541a(9) =15111a(10) =48854a(11) =154674a(12) =482355a(13) =1487905a(14) =4553684a(15) =13857492a(16) =41998265a(17) =126912075a(18) =382702050a(19) =1152300166a(20) =3465813071a(21) =10416313221a(22) =31288785152a(23) =93950241096a(24) =282026883573a(25) =846449748943