a(1) = 2; a(n) = 9*2^(n-2) - n - 2, n>1.

Terms

a(0) =2a(1) =5a(2) =13a(3) =30a(4) =65a(5) =136a(6) =279a(7) =566a(8) =1141a(9) =2292a(10) =4595a(11) =9202a(12) =18417a(13) =36848a(14) =73711a(15) =147438a(16) =294893a(17) =589804a(18) =1179627a(19) =2359274a(20) =4718569a(21) =9437160a(22) =18874343a(23) =37748710a(24) =75497445a(25) =150994916a(26) =301989859a(27) =603979746a(28) =1207959521