A056684 - OEIS

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A056684

Numbers k such that 50*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

3

11, 12, 608

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Also numbers k such that (5*10^(k+1)-41)/9 is prime.

a(4) > 10^5. - Robert Price, Nov 16 2014

LINKS

Table of n, a(n) for n=1..3.

Makoto Kamada, Prime numbers of the form 55...551.

Index entries for primes involving repunits

FORMULA

a(n) = A099415(n) - 1. - Robert Price, Nov 16 2014

MATHEMATICA

Do[ If[ PrimeQ[50*(10^n - 1)/9 + 1], Print[n]], {n, 15000}]

CROSSREFS

Cf. A002275, A099415.

Sequence in context: A262515 A041607 A042097 * A042731 A087304 A340204

Adjacent sequences: A056681 A056682 A056683 * A056685 A056686 A056687

KEYWORD

hard,nonn,bref,more

AUTHOR

Robert G. Wilson v, Aug 10 2000

STATUS

approved