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A256928
Numbers k such that R_(k+2) + 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 22, 280, 944, 1376, 1837, 2329, 2512, 5186, 24454, 25354, 49774, 103474, 159736, 177754
OFFSET
1,2
COMMENTS
Also, numbers k such that (136*10^k - 1)/9 is prime.
Terms from Kamada.
a(17) > 250000.
EXAMPLE
For k=2, R_4 + 4*10^2 = 1111 + 400 = 1511 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(136*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..300] | IsPrime((136*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((136*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
CROSSREFS
Cf. A002275.
Sequence in context: A135634 A156505 A367979 * A349107 A368447 A299824
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 13 2015
STATUS
approved