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 A284921 #11 Sep 08 2022 08:46:19
%S A284921 2,7,22,27,72,77,222,227,272,277,722,727,772,777,2222,2227,2272,2277,
%T A284921 2722,2727,2772,2777,7222,7227,7272,7277,7722,7727,7772,7777,22222,
%U A284921 22227,22272,22277,22722,22727,22772,22777,27222,27227,27272,27277,27722,27727
%N A284921 Numbers with digits 2 and 7 only.
%C A284921 Prime terms are in A020459.
%t A284921 Flatten@ Array[FromDigits /@ Tuples[{2, 7}, #] &, 5] (* _Michael De Vlieger_, Apr 06 2017 *)
%o A284921 (Magma) [n: n in [1..100000] | Set(IntegerToSequence(n, 10)) subset {2, 7}]
%Y A284921 Cf. Numbers with digits 2 and k only for k = 0 - 1 and 3 - 9: A169965 (k = 0), A007931 (k = 1), A032810 (k = 3), A284920 (k = 4), A072961 (k = 5), A284632 (k = 6), this sequence (k = 7), A284922 (k = 8), A284923 (k = 9).
%K A284921 nonn,base
%O A284921 1,1
%A A284921 _Jaroslav Krizek_, Apr 05 2017