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

Terms

a(0) =4a(1) =9a(2) =26a(3) =71a(4) =180a(5) =433a(6) =1006a(7) =2283a(8) =5096a(9) =11237a(10) =24546a(11) =53215a(12) =114652a(13) =245721a(14) =524246a(15) =1114067a(16) =2359248a(17) =4980685a(18) =10485706a(19) =22020039a(20) =46137284a(21) =96468929a(22) =201326526a(23) =419430331a(24) =872415160a(25) =1811939253a(26) =3758096306a(27) =7784628143a(28) =16106127276a(29) =33285996457