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A259123
Numbers k such that 4*R_k + 10^k - 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
1, 6, 66, 1254, 8406, 67038
OFFSET
1,2
COMMENTS
Also, numbers k such that (13*10^k - 31)/9 is prime.
Terms from Kamada data.
a(7) > 10^5.
EXAMPLE
For k=6, 4*R_6 + 10^k - 3 = 444444 + 1000000 - 3 = 1444441 which is prime.
MATHEMATICA
Select[Range[100000], PrimeQ[(13*10^#-31)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A329930 A367308 A271220 * A292026 A126459 A054969
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved