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A257025
Numbers k such that 3*R_(k+2) + 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
0, 1, 2, 5, 7, 14, 32, 65, 163, 398, 485, 1799, 1852, 3326, 3692, 7226, 12743, 15313, 110405, 120395, 132337, 140357, 153025, 194150
OFFSET
1,3
COMMENTS
Also, numbers k such that (112*10^k - 1)/3 is prime.
Terms from Kamada.
a(25) > 250000.
EXAMPLE
For k=2, 3*R_4 + 4*10^2 = 3333 + 400 = 3733 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(112*10^#-1)/3 ] &]
PROG
(PARI) for(n=0, 500, if(isprime((112*10^n-1)/3), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..300] | IsPrime((112*10^n-1) div 3)]; // Vincenzo Librandi, Apr 15 2015
CROSSREFS
Cf. A002275.
Sequence in context: A338231 A286573 A133511 * A076720 A245921 A215513
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
STATUS
approved