Number of equivalence classes in S_n under the {123, 321} {132, 231}-equivalence.

Terms

a(0) =4a(1) =8a(2) =18a(3) =36a(4) =88a(5) =176a(6) =470a(7) =940a(8) =2784a(9) =5568a(10) =18564a(11) =37128a(12) =140496a(13) =280992a(14) =1205406a(15) =2410812a(16) =11625040a(17) =23250080a(18) =124582172a(19) =249164344a(20) =1467151632a(21) =2934303264a(22) =18808816988a(23) =37617633976a(24) =260504960128a(25) =521009920256