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

Terms

a(0) =1a(1) =4a(2) =6a(3) =8a(4) =12a(5) =22a(6) =46a(7) =100a(8) =216a(9) =458a(10) =954a(11) =1960a(12) =3988a(13) =8062a(14) =16230a(15) =32588a(16) =65328a(17) =130834a(18) =261874a(19) =523984a(20) =1048236a(21) =2096774a(22) =4193886a(23) =8388148a(24) =16776712a(25) =33553882a(26) =67108266a(27) =134217080a(28) =268434756a(29) =536870158