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A257032
Numbers k such that 7*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
3, 9, 132, 8547, 13215, 14110
OFFSET
1,1
COMMENTS
Also, numbers k such that (691*10^k-7)/9 is prime.
Terms from Kamada.
a(7) > 30000.
EXAMPLE
For k=3, 7*R_5 - 10^3 = 77777 - 1000 = 76777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(691*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((691*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A082606 A062702 A289169 * A141143 A368092 A163402
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved