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A257027 Numbers k such that 7*R_(k+2) - 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
0, 2, 3, 9, 11, 18, 74, 131, 144, 161, 224, 282, 390, 398, 614, 791, 1313, 1866, 9708, 10544, 13292, 13394, 29703, 30779, 72446 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (646*10^n-7)/9 is prime.

Terms from Kamada.

a(26) > 10^5. Robert Price, Jul 31 2016

LINKS

Table of n, a(n) for n=1..25.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 717w.

Index entries for primes involving repunits.

EXAMPLE

For k=2, 7*R_4 - 6*10^2 = 7777 - 600 = 7177 which is prime.

a(1) =0 associated with 71. a(2)=2 associated with 7177. a(3)=3 associated with 71777. a(4) = 9 associated with 71777777777 etc . - Robert Price, Jul 31 2016

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(646*10^#-7)/9 ] &]

PROG

(PARI) for(n=0, 200, if(isprime((646*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

(MAGMA) [n: n in [0..300] | IsPrime((646*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275, A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A074338 A111319 A109658 * A271548 A110350 A057569

Adjacent sequences:  A257024 A257025 A257026 * A257028 A257029 A257030

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

EXTENSIONS

a(25) from Robert Price, Jul 31 2016

STATUS

approved

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Last modified November 18 19:59 EST 2019. Contains 329288 sequences. (Running on oeis4.)