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A257037 Numbers k such that 9*R_(k+2) - 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
1, 8, 9, 14, 54, 80, 487, 551, 600, 2502, 2544, 5593, 7949, 8635, 13407, 31128, 45504, 45933, 52303, 65121, 167501, 359354 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that 93*10^n-1 is prime.

Terms from Kamada.

a(23) > 400000.

LINKS

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Prime numbers of the form 9299...99.

Index entries for primes involving repunits.

EXAMPLE

For k=8, 9*R_10 - 7*10^8 = 9999999999 - 700000000 = 9299999999 which is prime.

MATHEMATICA

Select[Range[0, 400000], PrimeQ[93*10^#-1 ] &]

PROG

(MAGMA) [n: n in [0..400] | IsPrime(93*10^n-1)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275.

Sequence in context: A059869 A213016 A105833 * A114305 A101765 A256351

Adjacent sequences:  A257034 A257035 A257036 * A257038 A257039 A257040

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified December 10 23:33 EST 2018. Contains 318049 sequences. (Running on oeis4.)