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 A004725 #21 Apr 22 2021 22:09:21
%S A004725 0,1,2,3,4,5,7,8,9,10,11,12,13,14,15,1,17,18,19,20,21,22,23,24,25,2,
%T A004725 27,28,29,30,31,32,33,34,35,3,37,38,39,40,41,42,43,44,45,4,47,48,49,
%U A004725 50,51,52,53,54,55,5,57,58,59,0,1,2,3,4,5,7,8,9,70,71,72,73,74,75,7
%N A004725 Delete all 6's from the sequence of nonnegative integers.
%t A004725 d6[n_]:=Module[{c=DeleteCases[IntegerDigits[n],6]},If[c=={},Nothing, FromDigits[ c]]]; Array[d6,80,0] (* _Harvey P. Dale_, Oct 09 2017 *)
%o A004725 (Python)
%o A004725 def A004725(n):
%o A004725 l = len(str(n))
%o A004725 m = 2*(10**l-1)//3
%o A004725 k = n + l - int(n+l < m)
%o A004725 return 5 if k == m else int(str(k).replace('6','')) # _Chai Wah Wu_, Apr 20 2021
%Y A004725 Cf. A004719, A004720, A004721, A004722, A004723, A004724, A004726, A004727, A004728.
%K A004725 base,nonn
%O A004725 0,3
%A A004725 _N. J. A. Sloane_