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 A069531 #9 Aug 24 2019 11:52:40
%S A069531 1,0,0,0,0,0,3,0,0,0,1,0,3,0,0,0,8,0,9,0,0,0,11,0,0,0,0,0,14,0,0,0,0,
%T A069531 0,0,0,0,0,0,0,0,0,0,0,0,0,23,0,21,0,0,0,0,0,0,0,0,0,29,0,30,0,0,0,0,
%U A069531 0,0,0,0,0,0,0,4,0,0,0,3,0,0,0,0,0,0,0,0,0,0,0,22,0,3,0,0,0,0,0,48,0,0,0,2
%N A069531 Smallest positive k such that 10^k + 1 is divisible by n, or 0 if no such number exists.
%C A069531 When a(n) is not zero, it is a divisor of phi(n). If n is a prime with primitive root 10 (cf. A001913) then a(n) = (n-1)/2.
%H A069531 Antti Karttunen, <a href="/A069531/b069531.txt">Table of n, a(n) for n = 1..10001</a>
%e A069531 a(7) = a(13) = 3 as 1001 is divisible by 7 and 13. a(17) = 8 as 17 divides 100000001 = 10^8 + 1.
%o A069531 (PARI) A069531(n) = { fordiv(eulerphi(n),k,if(!((1+(10^k))%n),return(k))); (0); }; \\ _Antti Karttunen_, Aug 23 2019
%Y A069531 Cf. A069521 to A069530.
%Y A069531 Cf. A000010, A001913, A002329.
%K A069531 nonn
%O A069531 1,7
%A A069531 _Amarnath Murthy_, Apr 01 2002
%E A069531 More terms from _Vladeta Jovovic_, Apr 03 2002