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 A284973 #9 Sep 08 2022 08:46:19
%S A284973 4,9,44,49,94,99,444,449,494,499,944,949,994,999,4444,4449,4494,4499,
%T A284973 4944,4949,4994,4999,9444,9449,9494,9499,9944,9949,9994,9999,44444,
%U A284973 44449,44494,44499,44944,44949,44994,44999,49444,49449,49494,49499,49944,49949
%N A284973 Numbers with digits 4 and 9 only.
%o A284973 (Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {4, 9}]
%o A284973 (PARI) a(n,{p=[4,9]})={my(v=binary(n+1));fromdigits(vector(#v-1,i,p[2]*v[i+1]+p[1]*!v[i+1]))} \\ _R. J. Cano_, Apr 09 2017
%Y A284973 Prime terms are in A020466.
%Y A284973 Numbers with digits 4 and k only for k = 0 - 3 and 5 - 9: A169967 (k = 0), A032822 (k = 1), A284920 (k = 2), A032834 (k = 3), A256290 (k = 5), A284634 (k = 6), A284971 (k = 7), A284972 (k = 8), this sequence (k = 9).
%K A284973 nonn,base
%O A284973 1,1
%A A284973 _Jaroslav Krizek_, Apr 07 2017