a(n) = floor(2^n*(n/3 + 4/9)).

Terms

a(0) =0a(1) =1a(2) =4a(3) =11a(4) =28a(5) =67a(6) =156a(7) =355a(8) =796a(9) =1763a(10) =3868a(11) =8419a(12) =18204a(13) =39139a(14) =83740a(15) =178403a(16) =378652a(17) =800995a(18) =1689372a(19) =3553507a(20) =7456540a(21) =15612131a(22) =32622364a(23) =68040931a(24) =141674268a(25) =294533347a(26) =611436316a(27) =1267611875a(28) =2624702236a(29) =5428361443