a(n)=(s(n)+2)/4, where s(n)=n-th base 4 palindrome that starts with 2.

Terms

a(0) =1a(1) =3a(2) =9a(3) =10a(4) =11a(5) =12a(6) =33a(7) =38a(8) =43a(9) =48a(10) =129a(11) =133a(12) =137a(13) =141a(14) =146a(15) =150a(16) =154a(17) =158a(18) =163a(19) =167a(20) =171a(21) =175a(22) =180a(23) =184a(24) =188a(25) =192a(26) =513a(27) =533a(28) =553a(29) =573