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A257029
Numbers k such that 7*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
0, 66, 84, 3366, 14934
OFFSET
1,2
COMMENTS
Also, numbers k such that (664*10^k - 7)/9 is prime.
Terms from Kamada.
a(6) > 30000.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(664*10^#-7)/9 ] &]
PROG
(PARI) for(n=0, 100, if(isprime((664*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..400] | IsPrime((664*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A127654 A293175 A032485 * A223734 A031411 A098775
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved