A102985 Numbers n such that 4*10^n + 2*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

0, 1, 4, 9, 10, 31, 70, 160, 171, 277, 630, 724, 3717, 5542, 5634, 10656, 12724, 31954

Offset

1,3

Comments

Also numbers n such that (38*10^n+7)/9 is prime.

a(19) > 10^5. - Robert Price, May 09 2015

Links

Table of n, a(n) for n=1..18.

Makoto Kamada, Prime numbers of the form 422...223.

Index entries for primes involving repunits.

Formula

a(n) = A101720(n-1) + 1.

Mathematica

Do[ If[ PrimeQ[(38*10^n + 7)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101720.

Sequence in context: A141395 A121215 A178224 * A112401 A178360 A197125

Adjacent sequences: A102982 A102983 A102984 * A102986 A102987 A102988

Keyword

more,nonn

Author

Robert G. Wilson v, Jan 17 2005

Extensions

Addition of a(16)-a(17) from Kamada data by Robert Price, Dec 12 2010

a(18) from Erik Branger, May 01 2013, submitted by Ray Chandler, Aug 16 2013

Status

approved