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 A331815 #19 Feb 22 2020 22:11:22 %S A331815 3,4,132,471,1935,4258,9444 %N A331815 Numbers k such that 10^(2*k) - 8*10^(k-1) - 1 is prime. %C A331815 Also numbers k such that the concatenation (k 9's)1(k-1 9's) is prime. %H A331815 <a href="/index/Pri#primes_involving_repunits_.2C_sequences_related_to_">Index to OEIS entries related to primes involving repdigits</a>. %e A331815 3 is a term because 999199 is prime. %e A331815 4 is a term because 99991999 is prime. %t A331815 Select[Range[500], PrimeQ[10^(2*#) - 8*10^(#-1) - 1] &] (* _Amiram Eldar_, Jan 28 2020 *) %o A331815 (PARI) (is_A331815(n)=ispseudoprime(100^n-8*10^(n-1)-1)); for(n=1, 9999, is_A331815(n)&&print1(n", ")) %Y A331815 Cf. A000040. %Y A331815 Cf. A077776 = A183184*2+1: palindromic near-repdigit primes 9..919..9. %K A331815 nonn,base,more %O A331815 1,1 %A A331815 _Eder Vanzei_, Jan 27 2020 %E A331815 a(7) from _Giovanni Resta_, Jan 28 2020