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A257039
Numbers k such that 9*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
10, 12, 63, 69, 156, 328, 340, 344, 444, 672, 894, 1464, 1670, 1708, 2010, 4306, 7888, 8864, 9478, 9621, 26004, 36992, 71600, 98738, 118949, 130565, 140326, 183452, 211983, 225618
OFFSET
1,1
COMMENTS
Also, numbers k such that 96*10^k - 1 is prime.
Terms from Kamada.
846519 reported to Kamada by Bruno DallOsto is also in this sequence. It may or may not be a(31).
a(31) > 230000.
EXAMPLE
For k=10, 9*R_12 - 4*10^10 = 999999999999 - 40000000000 = 959999999999 which is prime.
MATHEMATICA
Select[Range[0, 230000], PrimeQ[96*10^#-1 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime(96*10^n-1)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A367149 A363285 A219917 * A337076 A223150 A333258
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved