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

Terms

a(0) =7a(1) =16a(2) =34a(3) =70a(4) =142a(5) =286a(6) =574a(7) =1150a(8) =2302a(9) =4606a(10) =9214a(11) =18430a(12) =36862a(13) =73726a(14) =147454a(15) =294910a(16) =589822a(17) =1179646a(18) =2359294a(19) =4718590a(20) =9437182a(21) =18874366a(22) =37748734a(23) =75497470a(24) =150994942a(25) =301989886a(26) =603979774a(27) =1207959550a(28) =2415919102