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A257026
Numbers k such that 3*R_(k+2) + 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 3, 9, 10, 13, 19, 25, 33, 124, 150, 175, 199, 290, 301, 506, 592, 1016, 1459, 4150, 6396, 7059, 8311, 8355, 8811, 13200, 13270, 26775, 33303, 62073, 66043, 154869, 162153, 172862
OFFSET
1,2
COMMENTS
Also, numbers k such that (115*10^k - 1)/3 is prime.
Terms from Kamada.
a(35) > 250000.
EXAMPLE
For k=2, 3*R_4 + 5*10^2 = 3333 + 500 = 3833 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(115*10^#-1)/3 ] &]
PROG
(PARI) for(n=0, 300, if(isprime((115*10^n-1)/3), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..300] | IsPrime((115*10^n-1) div 3)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A291924 A357353 A214433 * A057291 A135107 A191568
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved