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A256912
Numbers k such that R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 3, 4, 7, 10, 16, 66, 138, 5839, 23898, 112098
OFFSET
1,2
COMMENTS
Also, numbers k such that (91*10^k - 1)/9 is prime.
Terms from Kamada.
a(12) > 350000.
EXAMPLE
For k=3, R_5 - 10^3 = 11111 - 1000 = 10111 which is prime.
MATHEMATICA
Select[Range[0, 350000], PrimeQ[(91*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime((91*10^n - 1) div 9)]; // Vincenzo Librandi, Apr 13 2015
CROSSREFS
Cf. A002275.
Sequence in context: A241654 A047967 A282893 * A134591 A058611 A357459
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 12 2015
STATUS
approved