Multiply by 1, add 1, multiply by 2, add 2, etc., start with 1.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =6a(5) =18a(6) =21a(7) =84a(8) =88a(9) =440a(10) =445a(11) =2670a(12) =2676a(13) =18732a(14) =18739a(15) =149912a(16) =149920a(17) =1349280a(18) =1349289a(19) =13492890a(20) =13492900a(21) =148421900a(22) =148421911a(23) =1781062932a(24) =1781062944a(25) =23153818272a(26) =23153818285a(27) =324153455990a(28) =324153456004