A181607 - OEIS

%I #10 Jul 20 2019 01:05:27

%S 1,9,49,51,99,249,251,499,501,749,751,999,1249,3751,4999,5001,6249,

%T 8751,9999,18751,31249,49999,50001,68751,81249,99999,218751,281249,

%U 499999,500001,718751,781249,999999,4218751,4999999,5000001,5781249,9218751

%N Numbers n with k digits such that n^2 == 1 (mod 10^k).

%C Least term of n digits: 1, 49, 249, 1249, 18751, 218751, 4218751, ..., .

%C If n of k digits is present then 10^k-n is present.

%C 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 Eric M. Schmidt, <a href="/A181607/b181607.txt">Table of n, a(n) for n = 1..1000</a>

%t Table[ Select[ Range[10^(k - 1), 10^k - 1], Mod[ #^2, 10^k] == 1 &], {k, 7}] // Flatten

%Y Cf. A007185, A016090, A033819.

%K base,easy,nonn

%O 1,2

%A _Robert G. Wilson v_, Nov 01 2010