1 + (n+6)*2^(n-1).

Terms

a(0) =4a(1) =8a(2) =17a(3) =37a(4) =81a(5) =177a(6) =385a(7) =833a(8) =1793a(9) =3841a(10) =8193a(11) =17409a(12) =36865a(13) =77825a(14) =163841a(15) =344065a(16) =720897a(17) =1507329a(18) =3145729a(19) =6553601a(20) =13631489a(21) =28311553a(22) =58720257a(23) =121634817a(24) =251658241a(25) =520093697a(26) =1073741825a(27) =2214592513a(28) =4563402753a(29) =9395240961