A181607 Numbers n with k digits such that n^2 == 1 (mod 10^k).

1, 9, 49, 51, 99, 249, 251, 499, 501, 749, 751, 999, 1249, 3751, 4999, 5001, 6249, 8751, 9999, 18751, 31249, 49999, 50001, 68751, 81249, 99999, 218751, 281249, 499999, 500001, 718751, 781249, 999999, 4218751, 4999999, 5000001, 5781249, 9218751

Offset: 1

Author

Robert G. Wilson v, Nov 01 2010

Keywords

• base
• easy
• nonn

Comments

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

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

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

Links

• Eric M. Schmidt, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A007185, A016090, A033819.

Programs

Mathematica

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