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A056724 Numbers n such that 8*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
2, 9, 15, 32, 38, 65, 123, 173, 257, 320, 326, 639, 719, 774, 902, 15210, 69999 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers n such that (79*10^n-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: A319967 A063094 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

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Last modified May 18 00:43 EDT 2021. Contains 343992 sequences. (Running on oeis4.)