A055558 Primes of the form 1999...999.

19, 199, 1999, 199999, 19999999, 199999999999999999999999999, 1999999999999999999999999999, 199999999999999999999999999999999999999999999999999999

Offset: 1

Author

Labos Elemer, Jul 10 2000

Keywords

• base
• nonn

Comments

Primes of the form 2*10^k - 1.

Examples

			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.
		

Crossrefs

Subsequence of A090149.

Primes in A067272.

Cf. A002957, A051200.

Programs

PARI

for(n=1,99,if(ispseudoprime(t=2*10^n-1),print1(t", "))) \\ Charles R Greathouse IV, Dec 10 2013

Formula

a(n) = 2*10^A002957(n) - 1 = A067272(A002957(n) + 1). - Elmo R. Oliveira, Jun 14 2025