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A284636 Numbers with digits 6 and 9 only.

%I A284636 #21 Sep 08 2022 08:46:19
%S A284636 6,9,66,69,96,99,666,669,696,699,966,969,996,999,6666,6669,6696,6699,
%T A284636 6966,6969,6996,6999,9666,9669,9696,9699,9966,9969,9996,9999,66666,
%U A284636 66669,66696,66699,66966,66969,66996,66999,69666,69669,69696,69699,69966,69969
%N A284636 Numbers with digits 6 and 9 only.
%C A284636 All terms are composite.
%C A284636 All terms are divisible by 3. - _Michael S. Branicky_, Jun 09 2021
%H A284636 Felix Fröhlich, <a href="/A284636/b284636.txt">Table of n, a(n) for n = 1..10000</a>
%F A284636 a(n) = 3 * A032810(n).
%t A284636 Table[FromDigits /@ Tuples[{6, 9}, n], {n, 5}] // Flatten (* or *)
%t A284636 Select[Range@ 70000, Total@ Pick[DigitCount@ #, {0, 0, 0, 0, 0, 1, 0, 0, 1, 0}, 0] == 0 &] (* _Michael De Vlieger_, Apr 02 2017 *)
%o A284636 (Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {6, 9}]
%o A284636 (PARI)
%o A284636 a(n) = {
%o A284636   my(z, e = logint(n+1,2,&z),
%o A284636      t1 = 9 * subst(Pol(binary(n+1-z),'x), 'x, 10),
%o A284636      t2 = 6 * subst(Pol(binary(2*z-2-n),'x), 'x, 10));
%o A284636   t1+t2;
%o A284636 };
%o A284636 vector(44, n, a(n)) \\ _Gheorghe Coserea_, Apr 04 2017
%o A284636 (Python)
%o A284636 def a(n): return int(bin(n+1)[3:].replace('0', '6').replace('1', '9'))
%o A284636 print([a(n) for n in range(1, 45)]) # _Michael S. Branicky_, Jun 09 2021
%Y A284636 Cf. A032810.
%Y A284636 Numbers n with digits 6 and k only for k = 0 - 5 and 7 - 9: A204093 (k = 0), A284293 (k = 1), A284632 (k = 2), A284633 (k = 3), A284634 (k = 4), A256291 (k = 5), A256292 (k = 7), A284635 (k = 8), this sequence (k = 9).
%K A284636 nonn,base
%O A284636 1,1
%A A284636 _Jaroslav Krizek_, Apr 02 2017