a(n) = 9*2^n - 6.

Terms

a(0) =3a(1) =12a(2) =30a(3) =66a(4) =138a(5) =282a(6) =570a(7) =1146a(8) =2298a(9) =4602a(10) =9210a(11) =18426a(12) =36858a(13) =73722a(14) =147450a(15) =294906a(16) =589818a(17) =1179642a(18) =2359290a(19) =4718586a(20) =9437178a(21) =18874362a(22) =37748730a(23) =75497466a(24) =150994938a(25) =301989882a(26) =603979770a(27) =1207959546a(28) =2415919098a(29) =4831838202