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A257031 Numbers k such that 7*R_(k+2) - 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
1, 2, 5, 10, 11, 16, 23, 247, 1700, 2891, 3019, 5549, 5837, 9326, 14417, 23312, 24155, 30740 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (682*10^n-7)/9 is prime.

Terms from Kamada.

a(19) > 30000.

LINKS

Table of n, a(n) for n=1..18.

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

Makoto Kamada, Prime numbers of the form 7577...77.

Index entries for primes involving repunits.

EXAMPLE

For k=5, 7*R_7 - 2*10^5 = 7777777 - 200000 = 7577777 which is prime.

MATHEMATICA

Select[Range[0, 30000], PrimeQ[(682*10^#-7)/9 ] &]

PROG

(MAGMA) [n: n in [0..400] | IsPrime((682*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275.

Sequence in context: A056704 A230408 A163624 * A167799 A179871 A258116

Adjacent sequences:  A257028 A257029 A257030 * A257032 A257033 A257034

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified November 19 06:26 EST 2019. Contains 329310 sequences. (Running on oeis4.)