

A257031


Numbers k such that 7*R_(k+2)  2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n.


0



1, 2, 5, 10, 11, 16, 23, 247, 1700, 2891, 3019, 5549, 5837, 9326, 14417, 23312, 24155, 30740
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OFFSET

1,2


COMMENTS

Also, numbers k such that (682*10^n7)/9 is prime.
Terms from Kamada.
a(19) > 30000.


LINKS

Table of n, a(n) for n=1..18.
Makoto Kamada, Nearrepdigit numbers of the form ABAA...AA.
Makoto Kamada, Prime numbers of the form 7577...77.
Index entries for primes involving repunits.


EXAMPLE

For k=5, 7*R_7  2*10^5 = 7777777  200000 = 7577777 which is prime.


MATHEMATICA

Select[Range[0, 30000], PrimeQ[(682*10^#7)/9 ] &]


PROG

(MAGMA) [n: n in [0..400]  IsPrime((682*10^n7) div 9)]; // Vincenzo Librandi, Apr 15 2015


CROSSREFS

Cf. A002275.
Sequence in context: A056704 A230408 A163624 * A167799 A179871 A258116
Adjacent sequences: A257028 A257029 A257030 * A257032 A257033 A257034


KEYWORD

more,hard,nonn


AUTHOR

Robert Price, Apr 14 2015


STATUS

approved



