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A257028 Numbers k such that 7*R_(k+2) - 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
1, 4, 10, 28, 49, 64, 79, 109, 169, 1270, 5638, 6812, 7951, 11737, 16360, 22840, 25394 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (655*10^n-7)/9 is prime.

Terms from Kamada.

a(18) > 30000.

LINKS

Table of n, a(n) for n=1..17.

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

Makoto Kamada, Prime numbers of the form 7277...77.

Index entries for primes involving repunits.

EXAMPLE

For k=4, 7*R_6 - 5*10^2 = 777777 - 50000 = 727777 which is prime.

MATHEMATICA

Select[Range[0, 30000], PrimeQ[(655*10^#-7)/9 ] &]

PROG

(PARI) for(n=0, 200, if(isprime((655*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

(MAGMA) [n: n in [0..400] | IsPrime((655*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275.

Sequence in context: A222453 A186278 A222963 * A174938 A032248 A092504

Adjacent sequences:  A257025 A257026 A257027 * A257029 A257030 A257031

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified March 25 10:08 EDT 2019. Contains 321469 sequences. (Running on oeis4.)