Multiply by 1, add 1, multiply by 2, add 2, etc., starting with 0.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =4a(5) =12a(6) =15a(7) =60a(8) =64a(9) =320a(10) =325a(11) =1950a(12) =1956a(13) =13692a(14) =13699a(15) =109592a(16) =109600a(17) =986400a(18) =986409a(19) =9864090a(20) =9864100a(21) =108505100a(22) =108505111a(23) =1302061332a(24) =1302061344a(25) =16926797472a(26) =16926797485a(27) =236975164790a(28) =236975164804