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 A277550 #22 Feb 09 2021 02:26:07
%S A277550 1,5,6,11,16,21,25,26,30,31,36,41,46,51,55,56,61,66,71,76,80,81,86,91,
%T A277550 96,101,105,106,111,116,121,125,126,130,131,136,141,146,150,151,155,
%U A277550 156,161,166,171,176,180,181,186,191,196,201,205,206,211,216,221
%N A277550 Numbers k such that k/5^m == 1 (mod 5), where 5^m is the greatest power of 5 that divides k.
%C A277550 Positions of 1 in A277543. Numbers that have 1 as their rightmost nonzero digit when written in base 5.
%C A277550 This is one sequence in a 4-way splitting of the positive integers; the other three are indicated in the Mathematica program.
%H A277550 Clark Kimberling, <a href="/A277550/b277550.txt">Table of n, a(n) for n = 1..10000</a>
%t A277550 z = 200; a[b_] := Table[Mod[n/b^IntegerExponent[n, b], b], {n, 1, z}]
%t A277550 p[b_, d_] := Flatten[Position[a[b], d]]
%t A277550 p[5, 1] (* A277550 *)
%t A277550 p[5, 2] (* A277551 *)
%t A277550 p[5, 3] (* A277555 *)
%t A277550 p[5, 4] (* A277548 *)
%o A277550 (PARI) isok(n) = n/5^valuation(n, 5) % 5 == 1; \\ _Michel Marcus_, Oct 21 2016
%Y A277550 Cf. A277543, A277551, A277555, A277548.
%K A277550 nonn,easy
%O A277550 1,2
%A A277550 _Clark Kimberling_, Oct 20 2016