This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A289831 #23 Apr 22 2021 22:03:33
%S A289831 0,1,1,2,3,3,2,3,3,4,5,5,6,7,7,6,7,7,4,5,5,6,7,7,6,7,7,8,9,9,10,11,11,
%T A289831 10,11,11,12,13,13,14,15,15,14,15,15,12,13,13,14,15,15,14,15,15,8,9,9,
%U A289831 10,11,11,10,11,11,12,13,13,14,15,15,14,15,15,12
%N A289831 a(n) = A289813(n) + A289814(n).
%C A289831 The ones in the binary representation of a(n) correspond to the nonzero digits in the ternary representation of n; for example: ternary(42) = 1120 and binary(a(42)) = 1110 (a(42) = 14).
%C A289831 Each number k >= 0 appears 2^A000120(k) times.
%C A289831 a(A004488(n)) = a(n).
%H A289831 Rémy Sigrist, <a href="/A289831/b289831.txt">Table of n, a(n) for n = 0..6560</a>
%F A289831 a(0) = 0.
%F A289831 a(3*n) = 2*a(n).
%F A289831 a(3*n + 1) = 2*a(n) + 1.
%F A289831 a(3*n + 2) = 2*a(n) + 1.
%e A289831 The first values, alongside the ternary representation of n, and the binary representation of a(n), are:
%e A289831 n a(n) ternary(n) binary(a(n))
%e A289831 -- ---- ---------- ------------
%e A289831 0 0 0 0
%e A289831 1 1 1 1
%e A289831 2 1 2 1
%e A289831 3 2 10 10
%e A289831 4 3 11 11
%e A289831 5 3 12 11
%e A289831 6 2 20 10
%e A289831 7 3 21 11
%e A289831 8 3 22 11
%e A289831 9 4 100 100
%e A289831 10 5 101 101
%e A289831 11 5 102 101
%e A289831 12 6 110 110
%e A289831 13 7 111 111
%e A289831 14 7 112 111
%e A289831 15 6 120 110
%e A289831 16 7 121 111
%e A289831 17 7 122 111
%e A289831 18 4 200 100
%e A289831 19 5 201 101
%e A289831 20 5 202 101
%e A289831 21 6 210 110
%e A289831 22 7 211 111
%e A289831 23 7 212 111
%e A289831 24 6 220 110
%e A289831 25 7 221 111
%e A289831 26 7 222 111
%t A289831 Table[FromDigits[Sign@ IntegerDigits[n, 3], 2], {n, 0, 100}] (* _Indranil Ghosh_, Aug 03 2017 *)
%o A289831 (PARI) a(n) = my (d=digits(n,3)); fromdigits(vector(#d, i, sign(d[i])), 2)
%o A289831 (Python)
%o A289831 from sympy.ntheory.factor_ import digits
%o A289831 from sympy import sign
%o A289831 def a(n):
%o A289831 d=digits(n, 3)[1:]
%o A289831 return int(''.join(str(sign(i)) for i in d), 2)
%o A289831 print([a(n) for n in range(101)]) # _Indranil Ghosh_, Aug 03 2017
%Y A289831 Cf. A000120, A004488, A289813, A289814.
%K A289831 nonn,base
%O A289831 0,4
%A A289831 _Rémy Sigrist_, Jul 13 2017