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A256788
Numbers k such that 3*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 4, 16, 23, 25, 27, 29, 34, 68, 89, 127, 200, 418, 868, 1173, 2553, 4131, 5675, 8318, 8445, 10820, 15793, 16671, 32275, 43995, 48026, 50035, 88459, 126839
OFFSET
1,2
COMMENTS
Also, numbers k such that (106*10^k - 1)/3 is prime.
Terms from Kamada.
a(31) > 250000.
EXAMPLE
For k=4, 3*R_6 + 2*10^4 = 333333 + 20000 = 353333 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(106*10^#-1)/3 ] &]
CROSSREFS
Cf. A002275.
Sequence in context: A049441 A253143 A193868 * A324211 A059622 A131475
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved