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A099418 Numbers n such that 5*R_n + 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
2, 8, 12, 18, 26, 32, 138, 188, 222, 338, 1002, 2744, 6530, 38690, 39464 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also numbers n such that (5*10^n+31)/9 is prime.

LINKS

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

Makoto Kamada, Prime numbers of the form 55...559.

Index entries for primes involving repunits

FORMULA

a(n) = A056687(n) + 1.

MATHEMATICA

Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 4], Print[n]], {n, 0, 5000}]

CROSSREFS

Cf. A002275, A093403, A056687.

Sequence in context: A046525 A235353 A057265 * A108987 A035405 A280867

Adjacent sequences:  A099415 A099416 A099417 * A099419 A099420 A099421

KEYWORD

nonn

AUTHOR

Robert G. Wilson v, Oct 14 2004

EXTENSIONS

a(13)-a(15) from Kamada link by Ray Chandler, Feb 27 2012

STATUS

approved

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Last modified April 7 03:42 EDT 2020. Contains 333292 sequences. (Running on oeis4.)