a(0)=0, a(1)=1; thereafter a(n) = ceiling((3/2)^(n-3)*n*(n-1)).

Terms

a(0) =0a(1) =1a(2) =2a(3) =6a(4) =18a(5) =45a(6) =102a(7) =213a(8) =426a(9) =821a(10) =1538a(11) =2820a(12) =5075a(13) =8996a(14) =15743a(15) =27247a(16) =46709a(17) =79405a(18) =133996a(19) =224640a(20) =374400a(21) =620715a(22) =1024180a(23) =1682580a(24) =2753313a(25) =4489097a(26) =7294782a(27) =11817547a(28) =19089884a(29) =30755923