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A256773 Numbers k such that 3*R_k - 10 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
2, 4, 6, 10, 12, 14, 17, 41, 69, 141, 170, 188, 204, 233, 306, 555, 743, 1908, 2006, 2140, 2772, 8105, 15170, 40038, 41908, 48604, 51652, 54038, 56781, 129051 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (10^k - 31)/3 is prime.

Terms from Kamada data.

a(31) > 2.5*10^5.

LINKS

Table of n, a(n) for n=1..30.

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

Makoto Kamada, Prime numbers of the form 33...3323.

Index entries for primes involving repunits.

EXAMPLE

For k=4, 3*R_4 - 10 = 3333 - 10 = 3323 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(10^# - 31)/3] &]

PROG

(MAGMA) [n: n in [2..310] | IsPrime((10^n-31) div 3)]; // Vincenzo Librandi, Apr 10 2015

CROSSREFS

Cf. A002275.

Sequence in context: A022305 A103799 A175817 * A141104 A047410 A255056

Adjacent sequences:  A256770 A256771 A256772 * A256774 A256775 A256776

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Apr 09 2015

STATUS

approved

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Last modified January 20 17:47 EST 2018. Contains 297961 sequences.