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

1,3

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

Table of n, a(n) for n=1..31.

Makoto Kamada, Prime numbers of the form 166...667.

Index entries for primes involving repunits

Formula

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

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}]

Crossrefs

Cf. A000040, A002275, A102024, A126109.

Sequence in context: A350916 A014109 A328642 * A157049 A195668 A324698

Adjacent sequences: A102937 A102938 A102939 * A102941 A102942 A102943

Keyword

more,nonn

Author

Robert G. Wilson v, Dec 16 2004

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

Status

approved