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A259121
Numbers k such that 7*R_k + 10^k - 6 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
1, 6, 48, 102, 192, 366, 1002, 20364, 37446, 56082
OFFSET
1,2
COMMENTS
Also, numbers k such that (16*10^k - 61)/9 is prime.
Terms from Kamada data.
a(11) > 10^5.
EXAMPLE
For k=6, 7*R_6 + 10^k - 6 = 777777 + 10000000 - 6 = 1777771 which is prime.
MATHEMATICA
Select[Range[100000], PrimeQ[(16*10^#-61)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A078237 A274131 A341683 * A052651 A153796 A250226
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved