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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
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
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
Sequence in context: A359668 A191783 A358290 * A075372 A064725 A301761
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