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A259129
Numbers k such that 2*R_k + 3*10^k + 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
6, 8, 894, 1524, 3036, 21156
OFFSET
1,1
COMMENTS
Also, numbers k such that (29*10^k + 7)/9 is prime.
Terms from Kamada data.
a(7) > 10^5.
EXAMPLE
For k=6, 2*R_6 + 3*10^k + 1 = 222222 + 3000000 + 1 = 3222223 which is prime.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(29*10^#+7)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A000955 A027721 A264518 * A226472 A373702 A331480
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved