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A259120
Numbers k such that 9*R_k + 10^k - 8 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
1, 2, 4, 8, 40, 86, 200, 730, 1460, 23672, 28630
OFFSET
1,2
COMMENTS
Also, numbers k such that 2*10^k - 9 is prime.
Terms from Kamada data.
a(12) > 2*10^5.
EXAMPLE
For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.
MATHEMATICA
Select[Range[200000], PrimeQ[2*10^n-9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A172977 A018380 A058236 * A319595 A285632 A102918
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved