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A257030
Numbers k such that 7*R_(k+2) - 3*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 5, 23, 182, 209, 287, 7454, 16958
OFFSET
1,1
COMMENTS
Also, numbers k such that (673*10^k - 7)/9 is prime.
Terms from Kamada.
a(9) > 30000.
EXAMPLE
For k=5, 7*R_7 - 3*10^5 = 7777777 - 300000 = 7477777 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(673*10^#-7)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((673*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A343020 A167142 A136731 * A062495 A158889 A181074
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved