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A099416
Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 2, 8, 26, 66, 74, 233, 473, 540, 2774, 64715, 102492, 141594
OFFSET
1,2
COMMENTS
Also numbers k such that (5*10^k-23)/9 is prime.
a(12) > 10^5. - Robert Price, Nov 13 2014
FORMULA
a(n) = A056685(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]
CROSSREFS
Sequence in context: A216929 A100504 A327600 * A211885 A101696 A155861
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
a(11) from Robert Price, Nov 13 2014
0 removed by Georg Fischer, Jan 03 2021
a(12)-a(13) from Kamada data by Tyler Busby, May 03 2024
STATUS
approved