A098930 Numbers k such that 2*10^k + 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 3, 5, 6, 12, 186, 435, 1746, 3447, 18798, 70209

Offset

1,1

Comments

Also numbers k such that (23*10^k - 41)/9 is prime.

a(12) > 10^5. - Robert Price, Mar 04 2015

Links

Table of n, a(n) for n=1..11.

Makoto Kamada, Prime numbers of the form 255...551.

Index entries for primes involving repunits.

Formula

a(n) = A101961(n) + 1.

Example

If n = 3 we get 2551, which is prime.

Mathematica

Do[ If[ PrimeQ[(23*10^n - 41)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101961.

Sequence in context: A359668 A191783 A358290 * A075372 A064725 A301761

Adjacent sequences: A098927 A098928 A098929 * A098931 A098932 A098933

Keyword

more,nonn

Author

Julien Peter Benney (jpbenney(AT)ftml.net), Oct 20 2004

Extensions

a(7) from Ray Chandler, Nov 04 2004

a(8) & a(9) from Robert G. Wilson v, Dec 17 2004

Addition of a(10) from Kamada data by Robert Price, Dec 13 2010

a(11) from Robert Price, Mar 04 2015

Status

approved