Binary expansion contains 2 adjacent 1's.

Terms

a(0) =3a(1) =6a(2) =7a(3) =11a(4) =12a(5) =13a(6) =14a(7) =15a(8) =19a(9) =22a(10) =23a(11) =24a(12) =25a(13) =26a(14) =27a(15) =28a(16) =29a(17) =30a(18) =31a(19) =35a(20) =38a(21) =39a(22) =43a(23) =44a(24) =45a(25) =46a(26) =47a(27) =48a(28) =49a(29) =50