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 A062231 #19 Mar 24 2021 18:37:40
%S A062231 9,27,45,237
%N A062231 Numbers k such that the smoothly undulating palindromic number (97*10^k - 79)/99 is a prime.
%C A062231 Prime versus probable prime status and proofs are given in the author's table.
%C A062231 No further terms < 100000. - _Ray Chandler_, Aug 17 2011
%H A062231 Patrick De Geest, <a href="http://www.worldofnumbers.com/undulat.htm#supp979">SUPP Reference Table - 979</a>
%H A062231 <a href="/index/Pri#pri_supp">Index entries for sequences related to smoothly undulating palindromic primes</a>
%e A062231 k=27 -> (97*10^27 - 79)/99 = 979797979797979797979797979.
%t A062231 Select[Range[1,239,2],PrimeQ[FromDigits[PadRight[{},#,{9,7}]]]&] (* _Harvey P. Dale_, Mar 24 2021 *)
%Y A062231 Cf. A062209-A062232, A059758, A032758.
%K A062231 nonn,base,more
%O A062231 1,1
%A A062231 _Patrick De Geest_ and Hans Rosenthal (Hans.Rosenthal(AT)t-online.de), Jun 15 2001
%E A062231 Edited by _Ray Chandler_, Aug 17 2011