p-INVERT of the positive integers, where p(S) = (1 - S)^3.

Terms

a(0) =3a(1) =12a(2) =43a(3) =147a(4) =486a(5) =1566a(6) =4944a(7) =15351a(8) =47009a(9) =142278a(10) =426315a(11) =1266300a(12) =3732705a(13) =10928910a(14) =31806583a(15) =92069229a(16) =265215756a(17) =760621914a(18) =2172669846a(19) =6183333681a(20) =17538237677a(21) =49590486888a(22) =139817553417a(23) =393157465848