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 A191485 #20 Feb 26 2014 00:45:29 %S A191485 1,3,7,73,601,8191,262657,8640661 %N A191485 Numbers n=k^2-k+1 such that 2^k == 1 (mod n). %C A191485 The elements of this sequence are elements of the sequence A002061 (Central polygonal numbers). %C A191485 The first composite number is 8640661 = 31 * 211 * 1321 (31 and 211 are elements of the sequence A002061). %C A191485 No more terms up to 3773299855577673. %e A191485 k = 9; %e A191485 n = k^2 - k + 1 = 81 - 9 + 1 = 73; %e A191485 2^9 == 1 (mod 73). %o A191485 (PARI) for(k=1,10^9,n=k^2-k+1;if( lift(Mod(2,n)^k)==1,print1(n,", "))); /* _Joerg Arndt_, Jun 03 2011 */ %K A191485 nonn %O A191485 1,2 %A A191485 _Alzhekeyev Ascar M_, Jun 03 2011