The number of permutations of length n sortable by 3 reversals.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =118a(5) =534a(6) =1851a(7) =5158a(8) =12264a(9) =25943a(10) =50214a(11) =90656a(12) =154758a(13) =252304a(14) =395793a(15) =600894a(16) =886936a(17) =1277433a(18) =1800644a(19) =2490168a(20) =3385574a(21) =4533066a(22) =5986183a(23) =7806534a(24) =10064568a(25) =12840379a(26) =16224546a(27) =20319008a(28) =25237974a(29) =31108868