

A099419


Numbers n such that 7*R_n  6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.


2



2, 13, 20, 23, 31, 100, 241, 275, 925, 1067, 1369, 2065, 7163, 37963, 91856, 111706, 260198, 271757
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OFFSET

1,1


COMMENTS

Also numbers n such that (7*10^n61)/9 is prime.


LINKS

Table of n, a(n) for n=1..18.
Makoto Kamada, Prime numbers of the form 77...771.
Index entries for primes involving repunits


FORMULA

a(n) = A056688(n) + 1.  Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008


MATHEMATICA

Do[ If[ PrimeQ[ 7(10^n  1)/9  6], Print[n]], {n, 0, 7000}]


CROSSREFS

Cf. A093176.
Sequence in context: A007355 A105452 A219278 * A061871 A182459 A084651
Adjacent sequences: A099416 A099417 A099418 * A099420 A099421 A099422


KEYWORD

nonn


AUTHOR

Robert G. Wilson v, Oct 14 2004


EXTENSIONS

a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(14)a(15) from Erik Branger, Dec 19 2009
a(16) from Erik Branger, Jan 29 2011
a(17) from Erik Branger, Nov 26 2011
a(18) from Erik Branger, Mar 05 2012


STATUS

approved



