Numbers of the form 3^m - 1 or 2*3^m - 1; i.e., the union of sequences A048473 and A024023.

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =8a(5) =17a(6) =26a(7) =53a(8) =80a(9) =161a(10) =242a(11) =485a(12) =728a(13) =1457a(14) =2186a(15) =4373a(16) =6560a(17) =13121a(18) =19682a(19) =39365a(20) =59048a(21) =118097a(22) =177146a(23) =354293a(24) =531440a(25) =1062881a(26) =1594322a(27) =3188645a(28) =4782968a(29) =9565937