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 A321806 #31 Nov 21 2018 15:56:53
%S A321806 7,11,19,571,829
%N A321806 Number of zeros in palindromic strings of the form '123456789'|'1...0...1'|'987654321' whose decimal expansions are primes (palindromic).
%C A321806 Also numbers k such that 1234567891*10^(k+10) + 1987654321 is prime.
%C A321806 a(1)-a(5) are themselves primes i.e. numbers of (central) zeros in these palindromic strings (the decimal expansions of which are primes) are also primes.
%C A321806 If a(6) exists then it is >40000 (if it is composite) or >60000 (if it is prime).
%e A321806 a(1)=7 because the decimal expansion of the palindromic string '123456789100000001987654321' (concatenation of the prefix '123456789', the central string '100000001', and the suffix '987654321') is prime (palindromic) containing 7 zeros.
%e A321806 a(2)=11 as the decimal expansion of '1234567891000000000001987654321' is prime which contains 11 zeros.
%t A321806 ParallelMap[If[PrimeQ[1234567891*10^(#+10)+1987654321], #, Nothing] &, Range@1000]
%o A321806 (Perl) use ntheory "is_prime"; do { print "$_\n" if is_prime("1234567891" . "0"x$_ . "1987654321") } for 1..2000; # _Dana Jacobsen_, Nov 21 2018
%Y A321806 Cf. A002385.
%K A321806 nonn,base,hard,more
%O A321806 1,1
%A A321806 _Mikk Heidemaa_, Nov 19 2018