Dimension of n-th graded section of a certain Lie algebra.

Terms

a(0) =1a(1) =4a(2) =6a(3) =16a(4) =45a(5) =144a(6) =440a(7) =1440a(8) =4680a(9) =15600a(10) =52344a(11) =177840a(12) =608160a(13) =2095920a(14) =7262640a(15) =25300032a(16) =88517520a(17) =310927680a(18) =1095923400a(19) =3874804560a(20) =13737892896a(21) =48829153920a(22) =173949483240a(23) =620963048160