A103058 Numbers n such that 7*10^n + 4*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 2, 19, 31, 106, 26194, 39973

Offset

1,2

Comments

Also numbers n such that (67*10^n-13)/9 is prime.

a(8) > 10^5. - Robert Price, Sep 25 2015

Links

Table of n, a(n) for n=1..7.

Makoto Kamada, Prime numbers of the form 744...443.

Index entries for primes involving repunits.

Formula

a(n) = A101140(n) + 1.

Mathematica

Do[ If[ PrimeQ[(67*10^n - 13)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101140.

Sequence in context: A090819 A254897 A059697 * A161442 A135834 A106946

Adjacent sequences: A103055 A103056 A103057 * A103059 A103060 A103061

Keyword

more,nonn

Author

Robert G. Wilson v, Jan 19 2005

Extensions

a(6) from Kamada data by Robert Price, Dec 14 2010

a(7) from Robert Price, Sep 25 2015

Status

approved