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 A069858 #5 Jun 30 2017 11:48:17
%S A069858 101,202,303,404,505,606,808,909,1010,1111,1212,1515,1616,1818,2020,
%T A069858 2222,2424,2525,3030,3232,3333,3636,4040,4444,4545,4848,5050,5555,
%U A069858 6060,6464,6666,7272,7575,8080,8888,9090,9696,9999,10100,11110,12120,12625
%N A069858 1/n has period 4 in base 10.
%F A069858 Numbers of the form a*b*c where a is in {101, 303, 909, 1111, 3333, 9999}, b is a power of 2 and c is a power of 5.
%e A069858 1212 is in the sequence since 1/1212=0.0008250825082508250825082508250....
%t A069858 Select[Range[13000],Length[RealDigits[1/#][[1,-1]]]==4&] (* _Harvey P. Dale_, Jun 30 2017 *)
%Y A069858 Cf. A027894, A003592, A069105, A070021, A070022, A070023.
%K A069858 base,nonn
%O A069858 1,1
%A A069858 _Henry Bottomley_, May 02 2002