

A259121


Numbers k such that 7*R_k + 10^k  6 is prime, where R_k = 11...11 is the repunit (A002275) of length k.


0




OFFSET

1,2


COMMENTS

Also, numbers k such that (16*10^n61)/9 is prime.
Terms from Kamada data.
a(11) > 10^5.


LINKS

Table of n, a(n) for n=1..10.
Makoto Kamada, Nearrepdigit numbers of the form ABB...BBA.
Makoto Kamada, Prime numbers of the form 177...771.
Index entries for primes involving repunits.


EXAMPLE

For k=6, 7*R_6 + 10^k  6 = 777777 + 10000000  6 = 1777771 which is prime.


MATHEMATICA

Select[Range[100000], PrimeQ[(16*10^#61)/9] &]


CROSSREFS

Cf. A002275.
Sequence in context: A000252 A078237 A274131 * A052651 A153796 A250226
Adjacent sequences: A259118 A259119 A259120 * A259122 A259123 A259124


KEYWORD

nonn,more,hard


AUTHOR

Robert Price, Jun 18 2015


STATUS

approved



