

A098930


Numbers k such that 2*10^k + 5*R_k  4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.


1



2, 3, 5, 6, 12, 186, 435, 1746, 3447, 18798, 70209
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OFFSET

1,1


COMMENTS

Also numbers k such that (23*10^k  41)/9 is prime.
a(12) > 10^5.  Robert Price, Mar 04 2015


LINKS

Table of n, a(n) for n=1..11.
Makoto Kamada, Prime numbers of the form 255...551.
Index entries for primes involving repunits.


FORMULA

a(n) = A101961(n) + 1.


EXAMPLE

If n = 3 we get 2551, which is prime.


MATHEMATICA

Do[ If[ PrimeQ[(23*10^n  41)/9], Print[n]], {n, 0, 10000}]


CROSSREFS

Cf. A002275, A101961.
Sequence in context: A125877 A118787 A191783 * A075372 A064725 A301761
Adjacent sequences: A098927 A098928 A098929 * A098931 A098932 A098933


KEYWORD

more,nonn


AUTHOR

Julien Peter Benney (jpbenney(AT)ftml.net), Oct 20 2004


EXTENSIONS

a(7) from Ray Chandler, Nov 04 2004
a(8) & a(9) from Robert G. Wilson v, Dec 17 2004
Addition of a(10) from Kamada data by Robert Price, Dec 13 2010
a(11) from Robert Price, Mar 04 2015


STATUS

approved



