A093177 Primes of the form 90*R_k + 1, where R_k is the repunit (A002275) of length k.

991, 99991, 9999991, 999999999999999999999999999999991, 999999999999999999999999999999999999999999991, 999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999991

Offset: 1

Author

Rick L. Shepherd, Mar 27 2004

Keywords

• nonn

Comments

Primes of the form 10^k - 9 (k=3,5,7,...). - Vincenzo Librandi, Nov 16 2010

Links

• Makoto Kamada, Prime numbers of the form 99...991.
• Index entries for primes involving repunits

Crossrefs

Cf. A056696 (corresponding k), A095714.

Formula

a(n) = 10^(A056696(n)+1) - 9 = 10^A095714(n) - 9.

Extensions

Edited by Ray Chandler, Feb 26 2012