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A256458
Numbers k such that 3*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 4, 5, 6, 11, 26, 92, 98, 110, 215, 232, 354, 522, 648, 1862, 2322, 2934, 3797, 6017, 11236, 24748, 30011, 37864, 42904, 57414, 195478
OFFSET
1,1
COMMENTS
Also, numbers k such that (103*10^k - 1)/3 is prime.
Terms from Kamada.
a(27) > 250000.
EXAMPLE
For k=4, 3*R_6 + 10^4 = 333333 + 10000 = 343333 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(103*10^#-1)/3 ] &]
CROSSREFS
Cf. A002275.
Sequence in context: A194637 A194600 A255543 * A376702 A276001 A182109
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved