

A256912


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


0



1, 3, 4, 7, 10, 16, 66, 138, 5839, 23898, 112098
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OFFSET

1,2


COMMENTS

Also, numbers k such that (91*10^k  1)/9 is prime.
Terms from Kamada.
a(12) > 350000.


LINKS

Table of n, a(n) for n=1..11.
Makoto Kamada, Nearrepdigit numbers of the form ABAA...AA.
Makoto Kamada, Prime numbers of the form 1011...11.
Index entries for primes involving repunits.


EXAMPLE

For k=3, R_5  10^3 = 11111  1000 = 10111 which is prime.


MATHEMATICA

Select[Range[0, 350000], PrimeQ[(91*10^#1)/9 ] &]


PROG

(MAGMA) [n: n in [0..400]  IsPrime((91*10^n  1) div 9)]; // Vincenzo Librandi, Apr 13 2015


CROSSREFS

Cf. A002275.
Sequence in context: A241654 A047967 A282893 * A134591 A058611 A098613
Adjacent sequences: A256909 A256910 A256911 * A256913 A256914 A256915


KEYWORD

more,hard,nonn


AUTHOR

Robert Price, Apr 12 2015


STATUS

approved



