

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.


LINKS



FORMULA



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



KEYWORD

more,nonn


AUTHOR

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


EXTENSIONS

Addition of a(10) from Kamada data by Robert Price, Dec 13 2010


STATUS

approved



