a(n) = floor(2^n/9).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =7a(7) =14a(8) =28a(9) =56a(10) =113a(11) =227a(12) =455a(13) =910a(14) =1820a(15) =3640a(16) =7281a(17) =14563a(18) =29127a(19) =58254a(20) =116508a(21) =233016a(22) =466033a(23) =932067a(24) =1864135a(25) =3728270a(26) =7456540a(27) =14913080a(28) =29826161a(29) =59652323