a(n) = T(2, n), where T is the array given by A047858.

Terms

a(0) =1a(1) =4a(2) =11a(3) =27a(4) =63a(5) =143a(6) =319a(7) =703a(8) =1535a(9) =3327a(10) =7167a(11) =15359a(12) =32767a(13) =69631a(14) =147455a(15) =311295a(16) =655359a(17) =1376255a(18) =2883583a(19) =6029311a(20) =12582911a(21) =26214399a(22) =54525951a(23) =113246207a(24) =234881023a(25) =486539263a(26) =1006632959a(27) =2080374783a(28) =4294967295a(29) =8858370047