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 A240665 #7 Oct 24 2018 19:22:17
%S A240665 0,0,0,3,1,3,8,9,11,14,0,0,0,0,23,0,29,30,0,0,4,0,0,22,48,2,17,0,54,
%T A240665 56,21,65,4,23,74,0,39,0,83,0,89,90,0,96,49,0,15,111,0,114,116,0,15,
%U A240665 25,128,131,134,0,0,14,0,73,0,0,156,0,55,168,0,58,16,0
%N A240665 Least k such that 10^k == -1 (mod prime(n)), or 0 if no such k exists.
%C A240665 The least k, if it exists, such that prime(n) divides 10^k + 1.
%H A240665 T. D. Noe, <a href="/A240665/b240665.txt">Table of n, a(n) for n = 1..1000</a>
%F A240665 a(n) = A002371(n)/2 if A002371(n) is even, otherwise 0.
%F A240665 a(n) = A068958(n) for n > 3. - _Georg Fischer_, Oct 23 2018
%t A240665 Table[p = Prime[n]; s = Select[Range[p/2], PowerMod[10, #, p] == p - 1 &, 1]; If[s == {}, 0, s[[1]]], {n, 100}]
%Y A240665 Cf. A002371 (order of 10 mod prime(n)), A068958.
%K A240665 nonn
%O A240665 1,4
%A A240665 _T. D. Noe_, Apr 14 2014