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 A181607 #10 Jul 20 2019 01:05:27
%S A181607 1,9,49,51,99,249,251,499,501,749,751,999,1249,3751,4999,5001,6249,
%T A181607 8751,9999,18751,31249,49999,50001,68751,81249,99999,218751,281249,
%U A181607 499999,500001,718751,781249,999999,4218751,4999999,5000001,5781249,9218751
%N A181607 Numbers n with k digits such that n^2 == 1 (mod 10^k).
%C A181607 Least term of n digits: 1, 49, 249, 1249, 18751, 218751, 4218751, ..., .
%C A181607 If n of k digits is present then 10^k-n is present.
%C A181607 The union of A002283, A198971, A199685, A224473, A224474, A224475, and A224476 (except that this sequence omits 0, 4, and 6). - _Eric M. Schmidt_, Jan 26 2016
%H A181607 Eric M. Schmidt, <a href="/A181607/b181607.txt">Table of n, a(n) for n = 1..1000</a>
%t A181607 Table[ Select[ Range[10^(k - 1), 10^k - 1], Mod[ #^2, 10^k] == 1 &], {k, 7}] // Flatten
%Y A181607 Cf. A007185, A016090, A033819.
%K A181607 base,easy,nonn
%O A181607 1,2
%A A181607 _Robert G. Wilson v_, Nov 01 2010