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 A176351 #3 Mar 30 2012 17:27:06 %S A176351 4,180,320,5480,12096,17720,82780,1175232 %N A176351 Numbers n such that 2*3^n + 1 is a primitive prime factor of 10^3^n - 1. %C A176351 Consider the problem of finding the smallest number k such that the decimal representation of 1/k has period 3^e for a given e. The number k is usually 3^(e+2). However, if e is one of the n in this sequence, then the prime 2*3^n+1 is a smaller k. The first instance of these exceptions is 1/163, which has a period of 81. %C A176351 Subsequence of A003306. %C A176351 10 must be a square residue modulo 2*3^n + 1, implying that n must be a multiple of 4. %t A176351 Select[Range[10000], PrimeQ[1+2*3^# ] && MultiplicativeOrder[10,1+2*3^# ] == 3^# &] %Y A176351 Cf. A003306 (primes of the form 2*3^n+1), A003060 (least k such that 1/k has period n). %K A176351 hard,more,nonn %O A176351 1,1 %A A176351 _T. D. Noe_, Apr 15 2010 %E A176351 Two more terms from _Max Alekseyev_, May 03 2010