a(n) = Sum_{k=0..n} 2^(n-k) * floor(k/4).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =7a(7) =15a(8) =32a(9) =66a(10) =134a(11) =270a(12) =543a(13) =1089a(14) =2181a(15) =4365a(16) =8734a(17) =17472a(18) =34948a(19) =69900a(20) =139805a(21) =279615a(22) =559235a(23) =1118475a(24) =2236956a(25) =4473918a(26) =8947842a(27) =17895690a(28) =35791387a(29) =71582781