

A257030


Numbers k such that 7*R_(k+2)  3*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n.


0




OFFSET

1,1


COMMENTS

Also, numbers k such that (673*10^n7)/9 is prime.
Terms from Kamada.
a(9) > 30000.


LINKS

Table of n, a(n) for n=1..8.
Makoto Kamada, Nearrepdigit numbers of the form ABAA...AA.
Makoto Kamada, Prime numbers of the form 7477...77.
Index entries for primes involving repunits.


EXAMPLE

For k=5, 7*R_7  3*10^5 = 7777777  300000 = 7477777 which is prime.


MATHEMATICA

Select[Range[0, 30000], PrimeQ[(673*10^#7)/9 ] &]


PROG

(MAGMA) [n: n in [0..400]  IsPrime((673*10^n7) div 9)]; // Vincenzo Librandi, Apr 15 2015


CROSSREFS

Cf. A002275.
Sequence in context: A343020 A167142 A136731 * A062495 A158889 A181074
Adjacent sequences: A257027 A257028 A257029 * A257031 A257032 A257033


KEYWORD

more,hard,nonn


AUTHOR

Robert Price, Apr 14 2015


STATUS

approved



