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A259119
Numbers k such that 8*R_k + 10^k - 7 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
1, 2, 8, 14, 40, 92, 128, 884, 9424, 14768, 19258, 31234
OFFSET
1,2
COMMENTS
Also, numbers k such that (17*10^k - 71)/9 is prime.
Terms from Kamada data.
a(13) > 10^5.
EXAMPLE
For k=8, 8*R_8 + 10^k - 7 = 88888888 + 100000000 - 7 = 188888881 which is prime.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(17*10^#-71)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A332611 A162484 A333575 * A039660 A333053 A333054
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved