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A259120 Numbers k such that 9*R_k + 10^k - 8 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
1, 2, 4, 8, 40, 86, 200, 730, 1460, 23672, 28630 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that 2*10^k - 9 is prime.

Terms from Kamada data.

a(12) > 2*10^5.

LINKS

Table of n, a(n) for n=1..11.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 199...991.

Index entries for primes involving repunits.

EXAMPLE

For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.

MATHEMATICA

Select[Range[200000], PrimeQ[2*10^n-9] &]

CROSSREFS

Cf. A002275.

Sequence in context: A172977 A018380 A058236 * A319595 A285632 A102918

Adjacent sequences:  A259117 A259118 A259119 * A259121 A259122 A259123

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 18 2015

STATUS

approved

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Last modified May 19 05:04 EDT 2022. Contains 353826 sequences. (Running on oeis4.)