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 A217377 #6 Oct 03 2012 18:24:31 %S A217377 1,0,0,2,0,1,0,4,2,1,0,1,0,3,2,1,0,1,0,2,1,4,0,3,1,1,1,3,0,1,0,1,1,2, %T A217377 1,2,0,1,3,1,0,15,0,3,1,1,0,4,3,3008,1,1,0,2,1,1,4,1,0,3,0,1,1,2,2,1, %U A217377 0,1,3,1,0,1,0,2,2,1,1,4,0,2,1,4,0,5,2,8 %N A217377 a(n) is the smallest m>=0 such that ((5n+1)*6^m-1)/5 is prime; or -1 if no such value exists. %C A217377 Let f(n)=6n+1. Let f(n,m) be f applied to n m-times. For example f(n,3) = f(f(f(n))). Then a(n) is the smallest m>=0 such that f(n,m) is prime. %C A217377 a(525)=27871 is the largest found value in this sequence, which generates a probable prime with 21691 digits. %C A217377 a(1247) and a(1898) are currently unknown. If they are positive then a(1247)>86500 and a(1898)>58000. %H A217377 Dmitri Kamenetsky, <a href="/A217377/b217377.txt">Table of n, a(n) for n = 1..1246</a> %e A217377 a(8)=4, because 4 is the smallest value for m such that ((5*8+1)*6^m-1)/5 is prime. The prime value is (41*6^4-1)/5 = 6*(6*(6*(6*8+1)+1)+1)+1 = 10627. %Y A217377 Cf. A040081. %K A217377 nonn %O A217377 1,4 %A A217377 _Dmitri Kamenetsky_, Oct 01 2012