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A056686 Numbers n such that 50*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
0, 2, 3, 5, 9, 14, 21, 87, 206, 527, 959, 2099, 18521, 36782, 38516 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (5*10^(n+1)+13)/9 is prime.

a(16) > 10^5. - Robert Price, Nov 01 2014

LINKS

Table of n, a(n) for n=1..15.

Makoto Kamada, Prime numbers of the form 55...557.

Index entries for primes involving repunits

FORMULA

a(n) = A099417(n+1) - 1. - Robert Price, Nov 01 2014

MATHEMATICA

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

CROSSREFS

Cf. A002275, A099417.

Sequence in context: A076027 A280204 A240841 * A326332 A079962 A244986

Adjacent sequences:  A056683 A056684 A056685 * A056687 A056688 A056689

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

a(13)-a(15) derived from A099417 by Robert Price, Nov 01 2014

STATUS

approved

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Last modified August 25 03:10 EDT 2019. Contains 326318 sequences. (Running on oeis4.)