A102940 Numbers k such that 10^k + 6*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 3, 5, 9, 11, 14, 32, 54, 55, 60, 153, 200, 461, 569, 840, 847, 1296, 1356, 2007, 2627, 2847, 3110, 6876, 9161, 17765, 33555, 59142, 65773, 280710

Offset: 1

Author

Robert G. Wilson v, Dec 16 2004

Keywords

• more
• nonn

Comments

Also numbers k such that (5*10^k + 1)/3 is prime.

Numbers k such that A126109(k) is prime.

a(32) > 3*10^5. - Robert Price, Nov 15 2014

Links

• Makoto Kamada, Prime numbers of the form 166...667.
• Index entries for primes involving repunits

Crossrefs

Cf. A000040, A002275, A102024, A126109.

Programs

Maple

A102940:=n->`if`(isprime((5*10^n+1)/3),n,NULL): seq(A102940(n),n=0..1000); # Wesley Ivan Hurt, Nov 15 2014

Mathematica

Do[ If[ PrimeQ[(5*10^n + 1)/3], Print[n]], {n, 0, 10000}]

Formula

a(n) = A102024(n-1) + 1.

Extensions

Edited by N. J. A. Sloane, Mar 16 2007

Addition of a(27) from Kamada data by Robert Price, Dec 08 2010

a(28) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013

a(29)-a(31) from Kamada data by Robert Price, Nov 15 2014