A103054 Numbers n such that 7*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 3, 4, 7, 9, 13, 33, 37, 76, 147, 297, 1851, 3457, 3609, 45219

Offset

1,2

Comments

Also numbers n such that (65*10^n+61)/9 is prime.

a(16) > 10^5. - Robert Price, Oct 01 2015

Links

Table of n, a(n) for n=1..15.

Makoto Kamada, Prime numbers of the form 722...229.

Index entries for primes involving repunits.

Formula

a(n) = A101136(n) + 1.

Mathematica

Do[ If[ PrimeQ[(65*10^n + 61)/9], Print[n]], {n, 0, 10000}]
Select[Range[10000], PrimeQ[(65 10^# + 61)/9] &] (* Vincenzo Librandi, Oct 02 2015 *)

Prog

(Magma) [n: n in [0..3*10^2]| IsPrime((65*10^n+61) div 9)]; // Vincenzo Librandi, Oct 02 2015
(PARI) for(n=1, 1e4, if (isprime((65*10^n+61)/9), print1(n", "))) \\ Altug Alkan, Oct 02 2015

Crossrefs

Cf. A002275, A101136.

Sequence in context: A330146 A295069 A349360 * A140208 A098390 A266769

Adjacent sequences: A103051 A103052 A103053 * A103055 A103056 A103057

Keyword

more,nonn

Author

Robert G. Wilson v, Jan 19 2005

Extensions

a(15) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013

Status

approved