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 A161509 #9 Feb 10 2024 23:17:01
%S A161509 3,7,5,31,127,17,73,11,13,8191,43,151,257,131071,19,524287,41,337,683,
%T A161509 241,2731,262657,331,2147483647,65537,599479,43691,174763,61681,5419,
%U A161509 2796203,4432676798593,87211,15790321,2305843009213693951,715827883
%N A161509 The unique primitive prime factor of 2^n-1 for the n in A161508.
%C A161509 For these primes p, the binary expansion of 1/p has a unique period length. The binary analog of A007615.
%H A161509 Max Alekseyev, <a href="/A161509/b161509.txt">Table of n, a(n) for n = 1..179</a> (terms for n=1..100 from T. D. Noe)
%t A161509 Reap[Do[c=Cyclotomic[n,2]; q=c/GCD[c,n]; If[PrimePowerQ[q], Sow[FactorInteger[q][[1,1]]]],{n,100}]][[2,1]]
%Y A161509 Cf. A144755 (sorted).
%K A161509 nonn
%O A161509 1,1
%A A161509 _T. D. Noe_, Jun 17 2009