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

Terms

a(0) =-1a(1) =-1a(2) =0a(3) =0a(4) =2a(5) =6a(6) =9a(7) =36a(8) =40a(9) =200a(10) =205a(11) =1230a(12) =1236a(13) =8652a(14) =8659a(15) =69272a(16) =69280a(17) =623520a(18) =623529a(19) =6235290a(20) =6235300a(21) =68588300a(22) =68588311a(23) =823059732a(24) =823059744a(25) =10699776672a(26) =10699776685a(27) =149796873590a(28) =149796873604