Add 1, multiply by 1, add 2, multiply by 2, etc.; start with 0.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =6a(5) =9a(6) =27a(7) =31a(8) =124a(9) =129a(10) =645a(11) =651a(12) =3906a(13) =3913a(14) =27391a(15) =27399a(16) =219192a(17) =219201a(18) =1972809a(19) =1972819a(20) =19728190a(21) =19728201a(22) =217010211a(23) =217010223a(24) =2604122676a(25) =2604122689a(26) =33853594957a(27) =33853594971a(28) =473950329594a(29) =473950329609