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A256931
Numbers k such that R_(k+2) + 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
3, 16, 24, 165, 270, 478, 1683, 3976, 41608, 71760, 86982, 114226, 116610, 201504
OFFSET
1,1
COMMENTS
Also, numbers k such that (145*10^k - 1)/9 is prime.
Terms from Kamada.
a(15) > 250000.
EXAMPLE
For k=3, R_5 + 5*10^2 = 11111 + 5000 = 16111 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(145*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..300] | IsPrime((145*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 500, if(isprime((145*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
CROSSREFS
Cf. A002275.
Sequence in context: A101132 A153723 A091273 * A101405 A335308 A193367
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 13 2015
STATUS
approved