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A099410
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Numbers k such that 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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0, 2, 3, 5, 14, 176, 416, 2505, 2759, 7925, 9401, 10391, 12105, 19616
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (2*10^k + 61)/9 is prime.
Although perhaps a degenerate case, A002275 defines R(0)=0. Thus zero belongs in this sequence since 2*0+7=7 is prime. - Robert Price, Oct 28 2014
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LINKS
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FORMULA
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MATHEMATICA
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Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
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PROG
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(Magma) [n: n in [0..500] | IsPrime((2*10^n+61) div 9)]; // Vincenzo Librandi, Oct 30 2014
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Added zero and adapted Mathematica program by Robert Price, Oct 28 2014
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STATUS
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approved
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