a(n) = 63*2^n - 39 (n>=1).

Terms

a(0) =87a(1) =213a(2) =465a(3) =969a(4) =1977a(5) =3993a(6) =8025a(7) =16089a(8) =32217a(9) =64473a(10) =128985a(11) =258009a(12) =516057a(13) =1032153a(14) =2064345a(15) =4128729a(16) =8257497a(17) =16515033a(18) =33030105a(19) =66060249a(20) =132120537a(21) =264241113a(22) =528482265a(23) =1056964569a(24) =2113929177a(25) =4227858393a(26) =8455716825a(27) =16911433689a(28) =33822867417a(29) =67645734873