a(n) = 33*2^n - 45 (n>=1).

Terms

a(0) =21a(1) =87a(2) =219a(3) =483a(4) =1011a(5) =2067a(6) =4179a(7) =8403a(8) =16851a(9) =33747a(10) =67539a(11) =135123a(12) =270291a(13) =540627a(14) =1081299a(15) =2162643a(16) =4325331a(17) =8650707a(18) =17301459a(19) =34602963a(20) =69205971a(21) =138411987a(22) =276824019a(23) =553648083a(24) =1107296211a(25) =2214592467a(26) =4429184979a(27) =8858370003a(28) =17716740051a(29) =35433480147