

A099420


Numbers k such that 7*R_k  4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


3



1, 2, 3, 5, 9, 12, 15, 21, 264, 383, 2720, 4494, 21167, 45825, 55851, 64568, 70727
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OFFSET

1,2


COMMENTS

Also numbers k such that (7*10^k  43)/9 is prime.
a(18) > 10^5.  Robert Price, Oct 19 2014


LINKS

Table of n, a(n) for n=1..17.
Makoto Kamada, Prime numbers of the form 77...773.
Index entries for primes involving repunits


FORMULA

a(n) = A056689(n) + 1.


MATHEMATICA

Do[ If[ PrimeQ[ 7(10^n  1)/9  4], Print[n]], {n, 0, 5000}]


CROSSREFS

Cf. A002275, A056689, A093165.
Sequence in context: A141403 A124204 A161412 * A184794 A144728 A051147
Adjacent sequences: A099417 A099418 A099419 * A099421 A099422 A099423


KEYWORD

nonn,more


AUTHOR

Robert G. Wilson v, Oct 14 2004


EXTENSIONS

a(13)a(17) from Robert Price, Oct 19 2014


STATUS

approved



