a(n) = 9*2^n - 3*n - 5.

Terms

a(0) =4a(1) =10a(2) =25a(3) =58a(4) =127a(5) =268a(6) =553a(7) =1126a(8) =2275a(9) =4576a(10) =9181a(11) =18394a(12) =36823a(13) =73684a(14) =147409a(15) =294862a(16) =589771a(17) =1179592a(18) =2359237a(19) =4718530a(20) =9437119a(21) =18874300a(22) =37748665a(23) =75497398a(24) =150994867a(25) =301989808a(26) =603979693a(27) =1207959466a(28) =2415919015a(29) =4831838116