a(n) = 5*2^(n-1) - 2 - 3*n.

Terms

a(0) =0a(1) =2a(2) =9a(3) =26a(4) =63a(5) =140a(6) =297a(7) =614a(8) =1251a(9) =2528a(10) =5085a(11) =10202a(12) =20439a(13) =40916a(14) =81873a(15) =163790a(16) =327627a(17) =655304a(18) =1310661a(19) =2621378a(20) =5242815a(21) =10485692a(22) =20971449a(23) =41942966a(24) =83886003a(25) =167772080a(26) =335544237a(27) =671088554a(28) =1342177191