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 A055558 #21 Jun 14 2025 23:47:42
%S A055558 19,199,1999,199999,19999999,199999999999999999999999999,
%T A055558 1999999999999999999999999999,
%U A055558 199999999999999999999999999999999999999999999999999999
%N A055558 Primes of the form 1999...999.
%C A055558 Primes of the form 2*10^k - 1.
%F A055558 a(n) = 2*10^A002957(n) - 1 = A067272(A002957(n) + 1). - _Elmo R. Oliveira_, Jun 14 2025
%e A055558 2*10^n - 1 is prime for {1,2,3,5,7,26,27,53,147,236,248,386,401}; in each of these numbers, the digit '9' appears n times.
%o A055558 (PARI) for(n=1,99,if(ispseudoprime(t=2*10^n-1),print1(t", "))) \\ _Charles R Greathouse IV_, Dec 10 2013
%Y A055558 Subsequence of A090149.
%Y A055558 Primes in A067272.
%Y A055558 Cf. A002957, A051200.
%K A055558 base,nonn
%O A055558 1,1
%A A055558 _Labos Elemer_, Jul 10 2000