

A102931


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


2



1, 3, 6, 51, 57, 63, 150, 393, 420, 547, 42024, 43063
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OFFSET

1,2


COMMENTS

Also numbers k such that (11*10^k + 61)/9 is prime.
a(13) > 10^5.  Robert Price, Jan 17 2015


LINKS

Table of n, a(n) for n=1..12.
Makoto Kamada, Prime numbers of the form 122...229.
Index entries for primes involving repunits.


FORMULA

a(n) = A102011(n) + 1.


MATHEMATICA

Do[ If[ PrimeQ[(11*10^n + 61)/9], Print[n]], {n, 0, 10000}]


CROSSREFS

Cf. A002275, A102011.
Sequence in context: A032322 A203765 A111752 * A056447 A056437 A309320
Adjacent sequences: A102928 A102929 A102930 * A102932 A102933 A102934


KEYWORD

more,nonn


AUTHOR

Robert G. Wilson v, Dec 16 2004


EXTENSIONS

a(11)a(12) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013


STATUS

approved



