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 A246804 #27 Sep 08 2022 08:46:09 %S A246804 1,3,15,135,645,1373,195317,237249 %N A246804 Numbers k such that (10^(k+2) + 999) * 10^k + 1 is prime. %C A246804 Or, indices of primes in the sequence of decimal palindromes 19991, 1099901, 100999001, 10009990001, ... %C A246804 Or, numbers k such that there exists an "upside-down-Belphegor's primes" of length 2*k+3. %p A246804 A246804:=n->`if`(isprime((10^(n+2)+999)*10^n+1), n, NULL): seq(A246804(n), n=1..10^3); # _Wesley Ivan Hurt_, Nov 16 2014 %t A246804 Select[Range[10^3], PrimeQ[(10^(# + 2) + 999)*10^# + 1] &] %o A246804 (PARI) for( n=1,9999, ispseudoprime((10^(n+2)+999)*10^n+1) & print1(n",")) %o A246804 (Magma) [n: n in [1..500] | IsPrime((10^(n+2)+999)*10^n+1)]; %Y A246804 Cf. A156166 (Belphegor's primes), A082703 (plateau primes 199...991). %K A246804 nonn,more,base,hard %O A246804 1,2 %A A246804 _Serge Batalov_, Nov 16 2014