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A256828 Numbers k such that 7*R_k - 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
2, 6, 18, 48, 204, 10386, 19524, 28320 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (7*10^k - 367)/9 is prime.

Terms from Kamada.

a(9) > 30000.

LINKS

Table of n, a(n) for n=1..8.

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

Makoto Kamada, Prime numbers of the form 77...7737.

Index entries for primes involving repunits.

EXAMPLE

For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &]

PROG

(Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015

CROSSREFS

Cf. A002275.

Sequence in context: A218759 A295499 A059413 * A197055 A258625 A062026

Adjacent sequences: A256825 A256826 A256827 * A256829 A256830 A256831

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 10 2015

STATUS

approved

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Last modified January 31 18:32 EST 2023. Contains 359980 sequences. (Running on oeis4.)