A056724 Numbers k such that 8*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2, 9, 15, 32, 38, 65, 123, 173, 257, 320, 326, 639, 719, 774, 902, 15210, 69999

Offset

1,1

Comments

Also numbers k such that (79*10^k - 7)/9 is prime.

a(18) > 10^5. - Robert Price, Jan 11 2015

Links

Table of n, a(n) for n=1..17.

Makoto Kamada, Prime numbers of the form 877...77.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 8*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A093943.

Sequence in context: A063094 A369154 A108463 * A295732 A083174 A067547

Adjacent sequences: A056721 A056722 A056723 * A056725 A056726 A056727

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

a(16)-a(17) from Robert Price, Jan 11 2015

Status

approved