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 A186521 #24 Apr 03 2023 10:36:12 %S A186521 1,3,6,7,49,578,793,1322,1392,2392,3066,13479 %N A186521 Numbers n>0 such that (7*10^(n+2)+666)*10^n+7 is prime. %C A186521 Or, indices of primes in A046720. Rudolf Ondrejka calls these "beastly palindromic primes". %C A186521 Some of the larger entries may only correspond to probable primes. %H A186521 Chris Caldwell, The Prime Glossary, <a href="https://t5k.org/glossary/xpage/BeastlyPrime.html">Beastly prime</a> %H A186521 G. L. Honaker, Jr. and Chris Caldwell, <a href="https://t5k.org/curios/cpage/1032.html">Prime Curios! 76667</a> %H A186521 R. Ondrejka, <a href="http://www.utm.edu/research/primes/lists/top_ten/">The Top Ten: a Catalogue of Primal Configurations</a> %t A186521 Select[Range[10^3], PrimeQ[(7*10^(# + 2) + 666)*10^# + 7] &] (* _Arkadiusz Wesolowski_, Sep 08 2011 *) %Y A186521 Cf. A046720, A186086. %K A186521 nonn,base,more %O A186521 1,2 %A A186521 _Arkadiusz Wesolowski_, Feb 23 2011 %E A186521 a(12) added by _Arkadiusz Wesolowski_, Mar 16 2011