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A259132
Numbers k such that 5*R_k + 7*10^k + 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
2, 4, 10, 20, 22, 58, 74, 82, 208, 350, 422, 3812, 3982, 20924, 23786, 38852, 56042, 68504, 74434
OFFSET
1,1
COMMENTS
Also, numbers k such that (68*10^k + 13)/9 is prime.
Terms from Kamada data.
a(20) > 10^5.
EXAMPLE
For k=2, 5*R_2 + 7*10^k + 2 = 55 + 700 + 2 = 757 which is prime.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(68*10^#+13)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A011963 A083844 A026554 * A369391 A099413 A328761
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved