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A256722 Numbers k such that R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
2, 3, 5, 8, 9, 11, 18, 20, 29, 39, 123, 153, 267, 327, 627, 1020, 1686, 1764, 2960, 18236, 34613, 53991, 67404, 119880, 221654, 230706 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (10^k + 179)/9 is prime.

Terms from Kamada data.

a(27) > 2.5*10^5.

LINKS

Table of n, a(n) for n=1..26.

Makoto Kamada, Near-repdigit numbers of the form AA...AABA.

Makoto Kamada, Prime numbers of the form 11...1131.

Index entries for primes involving repunits.

EXAMPLE

For k=3, R_3 + 20 = 111 + 20 = 131 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(10^# + 179)/9] &]

PROG

(MAGMA) [n: n in [1..400] | IsPrime((10^n+179) div 9)]; // Vincenzo Librandi, Apr 10 2015

CROSSREFS

Cf. A002275.

Sequence in context: A226815 A284890 A051214 * A192391 A013634 A133484

Adjacent sequences:  A256719 A256720 A256721 * A256723 A256724 A256725

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 09 2015

STATUS

approved

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Last modified April 5 10:33 EDT 2020. Contains 333239 sequences. (Running on oeis4.)