Three even followed by one odd.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =2a(6) =2a(7) =3a(8) =4a(9) =4a(10) =4a(11) =5a(12) =6a(13) =6a(14) =6a(15) =7a(16) =8a(17) =8a(18) =8a(19) =9a(20) =10a(21) =10a(22) =10a(23) =11a(24) =12a(25) =12a(26) =12a(27) =13a(28) =14a(29) =14