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 A250288 #25 Dec 24 2014 23:29:43 %S A250288 7,13,17,37,47,73,101,131,151,167,197,241,263 %N A250288 Numbers n such that the duodecimal repunit (12^n - 1)/11 is a semiprime. %C A250288 First unknown term is 311. %C A250288 If (12^n - 1)/11 is a semiprime, n must be prime or the square of a prime (A001248), but no n = prime squared is known which yields a semiprime value of (12^n - 1)/11. (Specifically, n must be the square of a prime in A004064, and must be at least 491401 = 701^2.) %C A250288 No other known terms below 1000; the only other possible terms below 1000 are 449, 521, 571, 577, 613, 709, 751, 757, 769, 787, 853, 859, 887, 929, and 991. %H A250288 Samuel Wagstaff, <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a> %e A250288 a(1) = 7 so 1111111 = 46E * 2X3E (written in base 12). %t A250288 Select[Range[120], PrimeOmega[(12^# - 1)/11] == 2 &] (* _Alonso del Arte_, Dec 18 2014 *) %Y A250288 Cf. A046413, A085724, A004064. %K A250288 nonn,more %O A250288 1,1 %A A250288 _Eric Chen_, Dec 18 2014