

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
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OFFSET

1,2


COMMENTS

Also, numbers k such that (646*10^n7)/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 nearrepdigitrelated 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^n7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(MAGMA) [n: n in [0..300]  IsPrime((646*10^n7) 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



