a(n) = 2^n - 2^floor(2n/3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =12a(5) =24a(6) =48a(7) =112a(8) =224a(9) =448a(10) =960a(11) =1920a(12) =3840a(13) =7936a(14) =15872a(15) =31744a(16) =64512a(17) =129024a(18) =258048a(19) =520192a(20) =1040384a(21) =2080768a(22) =4177920a(23) =8355840a(24) =16711680a(25) =33488896a(26) =66977792a(27) =133955584a(28) =268173312a(29) =536346624