A103031 Numbers n such that 6*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

0, 3, 15, 339

Offset

1,2

Comments

Also numbers n such that (56*10^n+61)/9 is prime.

a(5) > 10^5. - Robert Price, Sep 07 2015

Links

Table of n, a(n) for n=1..4.

Makoto Kamada, Prime numbers of the form 622...229.

Index entries for primes involving repunits.

Formula

a(n) = A101524(n-1) + 1 for n>1.

Example

For n=0, (56*10^n+61)/9=13 which is prime, so 0 is in the sequence.

Mathematica

Do[ If[ PrimeQ[(56*10^n + 61)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A101524.

Sequence in context: A156769 A333691 A029758 * A338183 A012474 A336270

Adjacent sequences: A103028 A103029 A103030 * A103032 A103033 A103034

Keyword

bref,more,nonn

Author

Robert G. Wilson v, Jan 18 2005

Extensions

Inserted a(1)=0 by Robert Price, Sep 07 2015

Status

approved